- Copy stable/9 to releng/9.2 as part of the 9.2-RELEASE cycle.

[FreeBSD/releng/9.2.git] / sys / dev / xen / blkback / blkback.c

/*-
 * Copyright (c) 2009-2011 Spectra Logic Corporation
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions, and the following disclaimer,
 *    without modification.
 * 2. Redistributions in binary form must reproduce at minimum a disclaimer
 *    substantially similar to the "NO WARRANTY" disclaimer below
 *    ("Disclaimer") and any redistribution must be conditioned upon
 *    including a substantially similar Disclaimer requirement for further
 *    binary redistribution.
 *
 * NO WARRANTY
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDERS OR CONTRIBUTORS BE LIABLE FOR SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGES.
 *
 * Authors: Justin T. Gibbs     (Spectra Logic Corporation)
 *          Ken Merry           (Spectra Logic Corporation)
 */
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

/**
 * \file blkback.c
 *
 * \brief Device driver supporting the vending of block storage from
 *        a FreeBSD domain to other domains.
 */

#include "opt_kdtrace.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/malloc.h>

#include <sys/bio.h>
#include <sys/bus.h>
#include <sys/conf.h>
#include <sys/devicestat.h>
#include <sys/disk.h>
#include <sys/fcntl.h>
#include <sys/filedesc.h>
#include <sys/kdb.h>
#include <sys/module.h>
#include <sys/namei.h>
#include <sys/proc.h>
#include <sys/rman.h>
#include <sys/taskqueue.h>
#include <sys/types.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/sysctl.h>
#include <sys/bitstring.h>
#include <sys/sdt.h>

#include <geom/geom.h>

#include <machine/_inttypes.h>
#include <machine/xen/xen-os.h>

#include <vm/vm.h>
#include <vm/vm_extern.h>
#include <vm/vm_kern.h>

#include <xen/blkif.h>
#include <xen/evtchn.h>
#include <xen/gnttab.h>
#include <xen/xen_intr.h>

#include <xen/interface/event_channel.h>
#include <xen/interface/grant_table.h>

#include <xen/xenbus/xenbusvar.h>

/*--------------------------- Compile-time Tunables --------------------------*/
/**
 * The maximum number of outstanding request blocks (request headers plus
 * additional segment blocks) we will allow in a negotiated block-front/back
 * communication channel.
 */
#define XBB_MAX_REQUESTS        256

/**
 * \brief Define to force all I/O to be performed on memory owned by the
 *        backend device, with a copy-in/out to the remote domain's memory.
 *
 * \note  This option is currently required when this driver's domain is
 *        operating in HVM mode on a system using an IOMMU.
 *
 * This driver uses Xen's grant table API to gain access to the memory of
 * the remote domains it serves.  When our domain is operating in PV mode,
 * the grant table mechanism directly updates our domain's page table entries
 * to point to the physical pages of the remote domain.  This scheme guarantees
 * that blkback and the backing devices it uses can safely perform DMA
 * operations to satisfy requests.  In HVM mode, Xen may use a HW IOMMU to
 * insure that our domain cannot DMA to pages owned by another domain.  As
 * of Xen 4.0, IOMMU mappings for HVM guests are not updated via the grant
 * table API.  For this reason, in HVM mode, we must bounce all requests into
 * memory that is mapped into our domain at domain startup and thus has
 * valid IOMMU mappings.
 */
#define XBB_USE_BOUNCE_BUFFERS

/**
 * \brief Define to enable rudimentary request logging to the console.
 */
#undef XBB_DEBUG

/*---------------------------------- Macros ----------------------------------*/
/**
 * Custom malloc type for all driver allocations.
 */
static MALLOC_DEFINE(M_XENBLOCKBACK, "xbbd", "Xen Block Back Driver Data");

#ifdef XBB_DEBUG
#define DPRINTF(fmt, args...)                                   \
    printf("xbb(%s:%d): " fmt, __FUNCTION__, __LINE__, ##args)
#else
#define DPRINTF(fmt, args...) do {} while(0)
#endif

/**
 * The maximum mapped region size per request we will allow in a negotiated
 * block-front/back communication channel.
 */
#define XBB_MAX_REQUEST_SIZE                                    \
        MIN(MAXPHYS, BLKIF_MAX_SEGMENTS_PER_REQUEST * PAGE_SIZE)

/**
 * The maximum number of segments (within a request header and accompanying
 * segment blocks) per request we will allow in a negotiated block-front/back
 * communication channel.
 */
#define XBB_MAX_SEGMENTS_PER_REQUEST                            \
        (MIN(UIO_MAXIOV,                                        \
             MIN(BLKIF_MAX_SEGMENTS_PER_REQUEST,                \
                 (XBB_MAX_REQUEST_SIZE / PAGE_SIZE) + 1)))

/**
 * The maximum number of shared memory ring pages we will allow in a
 * negotiated block-front/back communication channel.  Allow enough
 * ring space for all requests to be XBB_MAX_REQUEST_SIZE'd.
 */
#define XBB_MAX_RING_PAGES                                                  \
        BLKIF_RING_PAGES(BLKIF_SEGS_TO_BLOCKS(XBB_MAX_SEGMENTS_PER_REQUEST) \
                       * XBB_MAX_REQUESTS)
/**
 * The maximum number of ring pages that we can allow per request list.
 * We limit this to the maximum number of segments per request, because
 * that is already a reasonable number of segments to aggregate.  This
 * number should never be smaller than XBB_MAX_SEGMENTS_PER_REQUEST,
 * because that would leave situations where we can't dispatch even one
 * large request.
 */
#define XBB_MAX_SEGMENTS_PER_REQLIST XBB_MAX_SEGMENTS_PER_REQUEST

/*--------------------------- Forward Declarations ---------------------------*/
struct xbb_softc;
struct xbb_xen_req;

static void xbb_attach_failed(struct xbb_softc *xbb, int err, const char *fmt,
                              ...) __attribute__((format(printf, 3, 4)));
static int  xbb_shutdown(struct xbb_softc *xbb);
static int  xbb_detach(device_t dev);

/*------------------------------ Data Structures -----------------------------*/

STAILQ_HEAD(xbb_xen_req_list, xbb_xen_req);

typedef enum {
        XBB_REQLIST_NONE        = 0x00,
        XBB_REQLIST_MAPPED      = 0x01
} xbb_reqlist_flags;

struct xbb_xen_reqlist {
        /**
         * Back reference to the parent block back instance for this
         * request.  Used during bio_done handling.
         */
        struct xbb_softc        *xbb;

        /**
         * BLKIF_OP code for this request.
         */
        int                      operation;

        /**
         * Set to BLKIF_RSP_* to indicate request status.
         *
         * This field allows an error status to be recorded even if the
         * delivery of this status must be deferred.  Deferred reporting
         * is necessary, for example, when an error is detected during
         * completion processing of one bio when other bios for this
         * request are still outstanding.
         */
        int                      status;

        /**
         * Number of 512 byte sectors not transferred.
         */
        int                      residual_512b_sectors;

        /**
         * Starting sector number of the first request in the list.
         */
        off_t                    starting_sector_number;

        /**
         * If we're going to coalesce, the next contiguous sector would be
         * this one.
         */
        off_t                    next_contig_sector;

        /**
         * Number of child requests in the list.
         */
        int                      num_children;

        /**
         * Number of I/O requests dispatched to the backend.
         */
        int                      pendcnt;

        /**
         * Total number of segments for requests in the list.
         */
        int                      nr_segments;

        /**
         * Flags for this particular request list.
         */
        xbb_reqlist_flags        flags;

        /**
         * Kernel virtual address space reserved for this request
         * list structure and used to map the remote domain's pages for
         * this I/O, into our domain's address space.
         */
        uint8_t                 *kva;

        /**
         * Base, psuedo-physical address, corresponding to the start
         * of this request's kva region.
         */
        uint64_t                 gnt_base;


#ifdef XBB_USE_BOUNCE_BUFFERS
        /**
         * Pre-allocated domain local memory used to proxy remote
         * domain memory during I/O operations.
         */
        uint8_t                 *bounce;
#endif

        /**
         * Array of grant handles (one per page) used to map this request.
         */
        grant_handle_t          *gnt_handles;

        /**
         * Device statistics request ordering type (ordered or simple).
         */
        devstat_tag_type         ds_tag_type;

        /**
         * Device statistics request type (read, write, no_data).
         */
        devstat_trans_flags      ds_trans_type;

        /**
         * The start time for this request.
         */
        struct bintime           ds_t0;

        /**
         * Linked list of contiguous requests with the same operation type.
         */
        struct xbb_xen_req_list  contig_req_list;

        /**
         * Linked list links used to aggregate idle requests in the
         * request list free pool (xbb->reqlist_free_stailq) and pending
         * requests waiting for execution (xbb->reqlist_pending_stailq).
         */
        STAILQ_ENTRY(xbb_xen_reqlist) links;
};

STAILQ_HEAD(xbb_xen_reqlist_list, xbb_xen_reqlist);

/**
 * \brief Object tracking an in-flight I/O from a Xen VBD consumer.
 */
struct xbb_xen_req {
        /**
         * Linked list links used to aggregate requests into a reqlist
         * and to store them in the request free pool.
         */
        STAILQ_ENTRY(xbb_xen_req) links;

        /**
         * The remote domain's identifier for this I/O request.
         */
        uint64_t                  id;

        /**
         * The number of pages currently mapped for this request.
         */
        int                       nr_pages;

        /**
         * The number of 512 byte sectors comprising this requests.
         */
        int                       nr_512b_sectors;

        /**
         * The number of struct bio requests still outstanding for this
         * request on the backend device.  This field is only used for  
         * device (rather than file) backed I/O.
         */
        int                       pendcnt;

        /**
         * BLKIF_OP code for this request.
         */
        int                       operation;

        /**
         * Storage used for non-native ring requests.
         */
        blkif_request_t          ring_req_storage;

        /**
         * Pointer to the Xen request in the ring.
         */
        blkif_request_t         *ring_req;

        /**
         * Consumer index for this request.
         */
        RING_IDX                 req_ring_idx;

        /**
         * The start time for this request.
         */
        struct bintime           ds_t0;

        /**
         * Pointer back to our parent request list.
         */
        struct xbb_xen_reqlist  *reqlist;
};
SLIST_HEAD(xbb_xen_req_slist, xbb_xen_req);

/**
 * \brief Configuration data for the shared memory request ring
 *        used to communicate with the front-end client of this
 *        this driver.
 */
struct xbb_ring_config {
        /** KVA address where ring memory is mapped. */
        vm_offset_t     va;

        /** The pseudo-physical address where ring memory is mapped.*/
        uint64_t        gnt_addr;

        /**
         * Grant table handles, one per-ring page, returned by the
         * hyperpervisor upon mapping of the ring and required to
         * unmap it when a connection is torn down.
         */
        grant_handle_t  handle[XBB_MAX_RING_PAGES];

        /**
         * The device bus address returned by the hypervisor when
         * mapping the ring and required to unmap it when a connection
         * is torn down.
         */
        uint64_t        bus_addr[XBB_MAX_RING_PAGES];

        /** The number of ring pages mapped for the current connection. */
        u_int           ring_pages;

        /**
         * The grant references, one per-ring page, supplied by the
         * front-end, allowing us to reference the ring pages in the
         * front-end's domain and to map these pages into our own domain.
         */
        grant_ref_t     ring_ref[XBB_MAX_RING_PAGES];

        /** The interrupt driven even channel used to signal ring events. */
        evtchn_port_t   evtchn;
};

/**
 * Per-instance connection state flags.
 */
typedef enum
{
        /**
         * The front-end requested a read-only mount of the
         * back-end device/file.
         */
        XBBF_READ_ONLY         = 0x01,

        /** Communication with the front-end has been established. */
        XBBF_RING_CONNECTED    = 0x02,

        /**
         * Front-end requests exist in the ring and are waiting for
         * xbb_xen_req objects to free up.
         */
        XBBF_RESOURCE_SHORTAGE = 0x04,

        /** Connection teardown in progress. */
        XBBF_SHUTDOWN          = 0x08,

        /** A thread is already performing shutdown processing. */
        XBBF_IN_SHUTDOWN       = 0x10
} xbb_flag_t;

/** Backend device type.  */
typedef enum {
        /** Backend type unknown. */
        XBB_TYPE_NONE           = 0x00,

        /**
         * Backend type disk (access via cdev switch
         * strategy routine).
         */
        XBB_TYPE_DISK           = 0x01,

        /** Backend type file (access vnode operations.). */
        XBB_TYPE_FILE           = 0x02
} xbb_type;

/**
 * \brief Structure used to memoize information about a per-request
 *        scatter-gather list.
 *
 * The chief benefit of using this data structure is it avoids having
 * to reparse the possibly discontiguous S/G list in the original
 * request.  Due to the way that the mapping of the memory backing an
 * I/O transaction is handled by Xen, a second pass is unavoidable.
 * At least this way the second walk is a simple array traversal.
 *
 * \note A single Scatter/Gather element in the block interface covers
 *       at most 1 machine page.  In this context a sector (blkif
 *       nomenclature, not what I'd choose) is a 512b aligned unit
 *       of mapping within the machine page referenced by an S/G
 *       element.
 */
struct xbb_sg {
        /** The number of 512b data chunks mapped in this S/G element. */
        int16_t nsect;

        /**
         * The index (0 based) of the first 512b data chunk mapped
         * in this S/G element.
         */
        uint8_t first_sect;

        /**
         * The index (0 based) of the last 512b data chunk mapped
         * in this S/G element.
         */
        uint8_t last_sect;
};

/**
 * Character device backend specific configuration data.
 */
struct xbb_dev_data {
        /** Cdev used for device backend access.  */
        struct cdev   *cdev;

        /** Cdev switch used for device backend access.  */
        struct cdevsw *csw;

        /** Used to hold a reference on opened cdev backend devices. */
        int            dev_ref;
};

/**
 * File backend specific configuration data.
 */
struct xbb_file_data {
        /** Credentials to use for vnode backed (file based) I/O. */
        struct ucred   *cred;

        /**
         * \brief Array of io vectors used to process file based I/O.
         *
         * Only a single file based request is outstanding per-xbb instance,
         * so we only need one of these.
         */
        struct iovec    xiovecs[XBB_MAX_SEGMENTS_PER_REQLIST];
#ifdef XBB_USE_BOUNCE_BUFFERS

        /**
         * \brief Array of io vectors used to handle bouncing of file reads.
         *
         * Vnode operations are free to modify uio data during their
         * exectuion.  In the case of a read with bounce buffering active,
         * we need some of the data from the original uio in order to
         * bounce-out the read data.  This array serves as the temporary
         * storage for this saved data.
         */
        struct iovec    saved_xiovecs[XBB_MAX_SEGMENTS_PER_REQLIST];

        /**
         * \brief Array of memoized bounce buffer kva offsets used
         *        in the file based backend.
         *
         * Due to the way that the mapping of the memory backing an
         * I/O transaction is handled by Xen, a second pass through
         * the request sg elements is unavoidable. We memoize the computed
         * bounce address here to reduce the cost of the second walk.
         */
        void            *xiovecs_vaddr[XBB_MAX_SEGMENTS_PER_REQLIST];
#endif /* XBB_USE_BOUNCE_BUFFERS */
};

/**
 * Collection of backend type specific data.
 */
union xbb_backend_data {
        struct xbb_dev_data  dev;
        struct xbb_file_data file;
};

/**
 * Function signature of backend specific I/O handlers.
 */
typedef int (*xbb_dispatch_t)(struct xbb_softc *xbb,
                              struct xbb_xen_reqlist *reqlist, int operation,
                              int flags);

/**
 * Per-instance configuration data.
 */
struct xbb_softc {

        /**
         * Task-queue used to process I/O requests.
         */
        struct taskqueue         *io_taskqueue;

        /**
         * Single "run the request queue" task enqueued
         * on io_taskqueue.
         */
        struct task               io_task;

        /** Device type for this instance. */
        xbb_type                  device_type;

        /** NewBus device corresponding to this instance. */
        device_t                  dev;

        /** Backend specific dispatch routine for this instance. */
        xbb_dispatch_t            dispatch_io;

        /** The number of requests outstanding on the backend device/file. */
        int                       active_request_count;

        /** Free pool of request tracking structures. */
        struct xbb_xen_req_list   request_free_stailq;

        /** Array, sized at connection time, of request tracking structures. */
        struct xbb_xen_req       *requests;

        /** Free pool of request list structures. */
        struct xbb_xen_reqlist_list reqlist_free_stailq;

        /** List of pending request lists awaiting execution. */
        struct xbb_xen_reqlist_list reqlist_pending_stailq;

        /** Array, sized at connection time, of request list structures. */
        struct xbb_xen_reqlist   *request_lists;

        /**
         * Global pool of kva used for mapping remote domain ring
         * and I/O transaction data.
         */
        vm_offset_t               kva;

        /** Psuedo-physical address corresponding to kva. */
        uint64_t                  gnt_base_addr;

        /** The size of the global kva pool. */
        int                       kva_size;

        /** The size of the KVA area used for request lists. */
        int                       reqlist_kva_size;

        /** The number of pages of KVA used for request lists */
        int                       reqlist_kva_pages;

        /** Bitmap of free KVA pages */
        bitstr_t                 *kva_free;

        /**
         * \brief Cached value of the front-end's domain id.
         * 
         * This value is used at once for each mapped page in
         * a transaction.  We cache it to avoid incuring the
         * cost of an ivar access every time this is needed.
         */
        domid_t                   otherend_id;

        /**
         * \brief The blkif protocol abi in effect.
         *
         * There are situations where the back and front ends can
         * have a different, native abi (e.g. intel x86_64 and
         * 32bit x86 domains on the same machine).  The back-end
         * always accomodates the front-end's native abi.  That
         * value is pulled from the XenStore and recorded here.
         */
        int                       abi;

        /**
         * \brief The maximum number of requests and request lists allowed
         *        to be in flight at a time.
         *
         * This value is negotiated via the XenStore.
         */
        u_int                     max_requests;

        /**
         * \brief The maximum number of segments (1 page per segment)
         *        that can be mapped by a request.
         *
         * This value is negotiated via the XenStore.
         */
        u_int                     max_request_segments;

        /**
         * \brief Maximum number of segments per request list.
         *
         * This value is derived from and will generally be larger than
         * max_request_segments.
         */
        u_int                     max_reqlist_segments;

        /**
         * The maximum size of any request to this back-end
         * device.
         *
         * This value is negotiated via the XenStore.
         */
        u_int                     max_request_size;

        /**
         * The maximum size of any request list.  This is derived directly
         * from max_reqlist_segments.
         */
        u_int                     max_reqlist_size;

        /** Various configuration and state bit flags. */
        xbb_flag_t                flags;

        /** Ring mapping and interrupt configuration data. */
        struct xbb_ring_config    ring_config;

        /** Runtime, cross-abi safe, structures for ring access. */
        blkif_back_rings_t        rings;

        /** IRQ mapping for the communication ring event channel. */
        int                       irq;

        /**
         * \brief Backend access mode flags (e.g. write, or read-only).
         *
         * This value is passed to us by the front-end via the XenStore.
         */
        char                     *dev_mode;

        /**
         * \brief Backend device type (e.g. "disk", "cdrom", "floppy").
         *
         * This value is passed to us by the front-end via the XenStore.
         * Currently unused.
         */
        char                     *dev_type;

        /**
         * \brief Backend device/file identifier.
         *
         * This value is passed to us by the front-end via the XenStore.
         * We expect this to be a POSIX path indicating the file or
         * device to open.
         */
        char                     *dev_name;

        /**
         * Vnode corresponding to the backend device node or file
         * we are acessing.
         */
        struct vnode             *vn;

        union xbb_backend_data    backend;

        /** The native sector size of the backend. */
        u_int                     sector_size;

        /** log2 of sector_size.  */
        u_int                     sector_size_shift;

        /** Size in bytes of the backend device or file.  */
        off_t                     media_size;

        /**
         * \brief media_size expressed in terms of the backend native
         *        sector size.
         *
         * (e.g. xbb->media_size >> xbb->sector_size_shift).
         */
        uint64_t                  media_num_sectors;

        /**
         * \brief Array of memoized scatter gather data computed during the
         *        conversion of blkif ring requests to internal xbb_xen_req
         *        structures.
         *
         * Ring processing is serialized so we only need one of these.
         */
        struct xbb_sg             xbb_sgs[XBB_MAX_SEGMENTS_PER_REQLIST];

        /**
         * Temporary grant table map used in xbb_dispatch_io().  When
         * XBB_MAX_SEGMENTS_PER_REQLIST gets large, keeping this on the
         * stack could cause a stack overflow.
         */
        struct gnttab_map_grant_ref   maps[XBB_MAX_SEGMENTS_PER_REQLIST];

        /** Mutex protecting per-instance data. */
        struct mtx                lock;

#ifdef XENHVM
        /**
         * Resource representing allocated physical address space
         * associated with our per-instance kva region.
         */
        struct resource          *pseudo_phys_res;

        /** Resource id for allocated physical address space. */
        int                       pseudo_phys_res_id;
#endif

        /**
         * I/O statistics from BlockBack dispatch down.  These are
         * coalesced requests, and we start them right before execution.
         */
        struct devstat           *xbb_stats;

        /**
         * I/O statistics coming into BlockBack.  These are the requests as
         * we get them from BlockFront.  They are started as soon as we
         * receive a request, and completed when the I/O is complete.
         */
        struct devstat           *xbb_stats_in;

        /** Disable sending flush to the backend */
        int                       disable_flush;

        /** Send a real flush for every N flush requests */
        int                       flush_interval;

        /** Count of flush requests in the interval */
        int                       flush_count;

        /** Don't coalesce requests if this is set */
        int                       no_coalesce_reqs;

        /** Number of requests we have received */
        uint64_t                  reqs_received;

        /** Number of requests we have completed*/
        uint64_t                  reqs_completed;

        /** How many forced dispatches (i.e. without coalescing) have happend */
        uint64_t                  forced_dispatch;

        /** How many normal dispatches have happend */
        uint64_t                  normal_dispatch;

        /** How many total dispatches have happend */
        uint64_t                  total_dispatch;

        /** How many times we have run out of KVA */
        uint64_t                  kva_shortages;

        /** How many times we have run out of request structures */
        uint64_t                  request_shortages;
};

/*---------------------------- Request Processing ----------------------------*/
/**
 * Allocate an internal transaction tracking structure from the free pool.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * \return  On success, a pointer to the allocated xbb_xen_req structure.
 *          Otherwise NULL.
 */
static inline struct xbb_xen_req *
xbb_get_req(struct xbb_softc *xbb)
{
        struct xbb_xen_req *req;

        req = NULL;

        mtx_assert(&xbb->lock, MA_OWNED);

        if ((req = STAILQ_FIRST(&xbb->request_free_stailq)) != NULL) {
                STAILQ_REMOVE_HEAD(&xbb->request_free_stailq, links);
                xbb->active_request_count++;
        }

        return (req);
}

/**
 * Return an allocated transaction tracking structure to the free pool.
 *
 * \param xbb  Per-instance xbb configuration structure.
 * \param req  The request structure to free.
 */
static inline void
xbb_release_req(struct xbb_softc *xbb, struct xbb_xen_req *req)
{
        mtx_assert(&xbb->lock, MA_OWNED);

        STAILQ_INSERT_HEAD(&xbb->request_free_stailq, req, links);
        xbb->active_request_count--;

        KASSERT(xbb->active_request_count >= 0,
                ("xbb_release_req: negative active count"));
}

/**
 * Return an xbb_xen_req_list of allocated xbb_xen_reqs to the free pool.
 *
 * \param xbb       Per-instance xbb configuration structure.
 * \param req_list  The list of requests to free.
 * \param nreqs     The number of items in the list.
 */
static inline void
xbb_release_reqs(struct xbb_softc *xbb, struct xbb_xen_req_list *req_list,
                 int nreqs)
{
        mtx_assert(&xbb->lock, MA_OWNED);

        STAILQ_CONCAT(&xbb->request_free_stailq, req_list);
        xbb->active_request_count -= nreqs;

        KASSERT(xbb->active_request_count >= 0,
                ("xbb_release_reqs: negative active count"));
}

/**
 * Given a page index and 512b sector offset within that page,
 * calculate an offset into a request's kva region.
 *
 * \param reqlist The request structure whose kva region will be accessed.
 * \param pagenr  The page index used to compute the kva offset.
 * \param sector  The 512b sector index used to compute the page relative
 *                kva offset.
 *
 * \return  The computed global KVA offset.
 */
static inline uint8_t *
xbb_reqlist_vaddr(struct xbb_xen_reqlist *reqlist, int pagenr, int sector)
{
        return (reqlist->kva + (PAGE_SIZE * pagenr) + (sector << 9));
}

#ifdef XBB_USE_BOUNCE_BUFFERS
/**
 * Given a page index and 512b sector offset within that page,
 * calculate an offset into a request's local bounce memory region.
 *
 * \param reqlist The request structure whose bounce region will be accessed.
 * \param pagenr  The page index used to compute the bounce offset.
 * \param sector  The 512b sector index used to compute the page relative
 *                bounce offset.
 *
 * \return  The computed global bounce buffer address.
 */
static inline uint8_t *
xbb_reqlist_bounce_addr(struct xbb_xen_reqlist *reqlist, int pagenr, int sector)
{
        return (reqlist->bounce + (PAGE_SIZE * pagenr) + (sector << 9));
}
#endif

/**
 * Given a page number and 512b sector offset within that page,
 * calculate an offset into the request's memory region that the
 * underlying backend device/file should use for I/O.
 *
 * \param reqlist The request structure whose I/O region will be accessed.
 * \param pagenr  The page index used to compute the I/O offset.
 * \param sector  The 512b sector index used to compute the page relative
 *                I/O offset.
 *
 * \return  The computed global I/O address.
 *
 * Depending on configuration, this will either be a local bounce buffer
 * or a pointer to the memory mapped in from the front-end domain for
 * this request.
 */
static inline uint8_t *
xbb_reqlist_ioaddr(struct xbb_xen_reqlist *reqlist, int pagenr, int sector)
{
#ifdef XBB_USE_BOUNCE_BUFFERS
        return (xbb_reqlist_bounce_addr(reqlist, pagenr, sector));
#else
        return (xbb_reqlist_vaddr(reqlist, pagenr, sector));
#endif
}

/**
 * Given a page index and 512b sector offset within that page, calculate
 * an offset into the local psuedo-physical address space used to map a
 * front-end's request data into a request.
 *
 * \param reqlist The request list structure whose pseudo-physical region
 *                will be accessed.
 * \param pagenr  The page index used to compute the pseudo-physical offset.
 * \param sector  The 512b sector index used to compute the page relative
 *                pseudo-physical offset.
 *
 * \return  The computed global pseudo-phsyical address.
 *
 * Depending on configuration, this will either be a local bounce buffer
 * or a pointer to the memory mapped in from the front-end domain for
 * this request.
 */
static inline uintptr_t
xbb_get_gntaddr(struct xbb_xen_reqlist *reqlist, int pagenr, int sector)
{
        struct xbb_softc *xbb;

        xbb = reqlist->xbb;

        return ((uintptr_t)(xbb->gnt_base_addr +
                (uintptr_t)(reqlist->kva - xbb->kva) +
                (PAGE_SIZE * pagenr) + (sector << 9)));
}

/**
 * Get Kernel Virtual Address space for mapping requests.
 *
 * \param xbb         Per-instance xbb configuration structure.
 * \param nr_pages    Number of pages needed.
 * \param check_only  If set, check for free KVA but don't allocate it.
 * \param have_lock   If set, xbb lock is already held.
 *
 * \return  On success, a pointer to the allocated KVA region.  Otherwise NULL.
 *
 * Note:  This should be unnecessary once we have either chaining or
 * scatter/gather support for struct bio.  At that point we'll be able to
 * put multiple addresses and lengths in one bio/bio chain and won't need
 * to map everything into one virtual segment.
 */
static uint8_t *
xbb_get_kva(struct xbb_softc *xbb, int nr_pages)
{
        intptr_t first_clear;
        intptr_t num_clear;
        uint8_t *free_kva;
        int      i;

        KASSERT(nr_pages != 0, ("xbb_get_kva of zero length"));

        first_clear = 0;
        free_kva = NULL;

        mtx_lock(&xbb->lock);

        /*
         * Look for the first available page.  If there are none, we're done.
         */
        bit_ffc(xbb->kva_free, xbb->reqlist_kva_pages, &first_clear);

        if (first_clear == -1)
                goto bailout;

        /*
         * Starting at the first available page, look for consecutive free
         * pages that will satisfy the user's request.
         */
        for (i = first_clear, num_clear = 0; i < xbb->reqlist_kva_pages; i++) {
                /*
                 * If this is true, the page is used, so we have to reset
                 * the number of clear pages and the first clear page
                 * (since it pointed to a region with an insufficient number
                 * of clear pages).
                 */
                if (bit_test(xbb->kva_free, i)) {
                        num_clear = 0;
                        first_clear = -1;
                        continue;
                }

                if (first_clear == -1)
                        first_clear = i;

                /*
                 * If this is true, we've found a large enough free region
                 * to satisfy the request.
                 */
                if (++num_clear == nr_pages) {

                        bit_nset(xbb->kva_free, first_clear,
                                 first_clear + nr_pages - 1);

                        free_kva = xbb->kva +
                                (uint8_t *)(first_clear * PAGE_SIZE);

                        KASSERT(free_kva >= (uint8_t *)xbb->kva &&
                                free_kva + (nr_pages * PAGE_SIZE) <=
                                (uint8_t *)xbb->ring_config.va,
                                ("Free KVA %p len %d out of range, "
                                 "kva = %#jx, ring VA = %#jx\n", free_kva,
                                 nr_pages * PAGE_SIZE, (uintmax_t)xbb->kva,
                                 (uintmax_t)xbb->ring_config.va));
                        break;
                }
        }

bailout:

        if (free_kva == NULL) {
                xbb->flags |= XBBF_RESOURCE_SHORTAGE;
                xbb->kva_shortages++;
        }

        mtx_unlock(&xbb->lock);

        return (free_kva);
}

/**
 * Free allocated KVA.
 *
 * \param xbb       Per-instance xbb configuration structure.
 * \param kva_ptr   Pointer to allocated KVA region.  
 * \param nr_pages  Number of pages in the KVA region.
 */
static void
xbb_free_kva(struct xbb_softc *xbb, uint8_t *kva_ptr, int nr_pages)
{
        intptr_t start_page;

        mtx_assert(&xbb->lock, MA_OWNED);

        start_page = (intptr_t)(kva_ptr - xbb->kva) >> PAGE_SHIFT;
        bit_nclear(xbb->kva_free, start_page, start_page + nr_pages - 1);

}

/**
 * Unmap the front-end pages associated with this I/O request.
 *
 * \param req  The request structure to unmap.
 */
static void
xbb_unmap_reqlist(struct xbb_xen_reqlist *reqlist)
{
        struct gnttab_unmap_grant_ref unmap[XBB_MAX_SEGMENTS_PER_REQLIST];
        u_int                         i;
        u_int                         invcount;
        int                           error;

        invcount = 0;
        for (i = 0; i < reqlist->nr_segments; i++) {

                if (reqlist->gnt_handles[i] == GRANT_REF_INVALID)
                        continue;

                unmap[invcount].host_addr    = xbb_get_gntaddr(reqlist, i, 0);
                unmap[invcount].dev_bus_addr = 0;
                unmap[invcount].handle       = reqlist->gnt_handles[i];
                reqlist->gnt_handles[i]      = GRANT_REF_INVALID;
                invcount++;
        }

        error = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref,
                                          unmap, invcount);
        KASSERT(error == 0, ("Grant table operation failed"));
}

/**
 * Allocate an internal transaction tracking structure from the free pool.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * \return  On success, a pointer to the allocated xbb_xen_reqlist structure.
 *          Otherwise NULL.
 */
static inline struct xbb_xen_reqlist *
xbb_get_reqlist(struct xbb_softc *xbb)
{
        struct xbb_xen_reqlist *reqlist;

        reqlist = NULL;

        mtx_assert(&xbb->lock, MA_OWNED);

        if ((reqlist = STAILQ_FIRST(&xbb->reqlist_free_stailq)) != NULL) {

                STAILQ_REMOVE_HEAD(&xbb->reqlist_free_stailq, links);
                reqlist->flags = XBB_REQLIST_NONE;
                reqlist->kva = NULL;
                reqlist->status = BLKIF_RSP_OKAY;
                reqlist->residual_512b_sectors = 0;
                reqlist->num_children = 0;
                reqlist->nr_segments = 0;
                STAILQ_INIT(&reqlist->contig_req_list);
        }

        return (reqlist);
}

/**
 * Return an allocated transaction tracking structure to the free pool.
 *
 * \param xbb        Per-instance xbb configuration structure.
 * \param req        The request list structure to free.
 * \param wakeup     If set, wakeup the work thread if freeing this reqlist
 *                   during a resource shortage condition.
 */
static inline void
xbb_release_reqlist(struct xbb_softc *xbb, struct xbb_xen_reqlist *reqlist,
                    int wakeup)
{

        mtx_lock(&xbb->lock);

        if (wakeup) {
                wakeup = xbb->flags & XBBF_RESOURCE_SHORTAGE;
                xbb->flags &= ~XBBF_RESOURCE_SHORTAGE;
        }

        if (reqlist->kva != NULL)
                xbb_free_kva(xbb, reqlist->kva, reqlist->nr_segments);

        xbb_release_reqs(xbb, &reqlist->contig_req_list, reqlist->num_children);

        STAILQ_INSERT_TAIL(&xbb->reqlist_free_stailq, reqlist, links);

        if ((xbb->flags & XBBF_SHUTDOWN) != 0) {
                /*
                 * Shutdown is in progress.  See if we can
                 * progress further now that one more request
                 * has completed and been returned to the
                 * free pool.
                 */
                xbb_shutdown(xbb);
        }

        mtx_unlock(&xbb->lock);

        if (wakeup != 0)
                taskqueue_enqueue(xbb->io_taskqueue, &xbb->io_task); 
}

/**
 * Request resources and do basic request setup.
 *
 * \param xbb          Per-instance xbb configuration structure.
 * \param reqlist      Pointer to reqlist pointer.
 * \param ring_req     Pointer to a block ring request.
 * \param ring_index   The ring index of this request.
 *
 * \return  0 for success, non-zero for failure.
 */
static int
xbb_get_resources(struct xbb_softc *xbb, struct xbb_xen_reqlist **reqlist,
                  blkif_request_t *ring_req, RING_IDX ring_idx)
{
        struct xbb_xen_reqlist *nreqlist;
        struct xbb_xen_req     *nreq;

        nreqlist = NULL;
        nreq     = NULL;

        mtx_lock(&xbb->lock);

        /*
         * We don't allow new resources to be allocated if we're in the
         * process of shutting down.
         */
        if ((xbb->flags & XBBF_SHUTDOWN) != 0) {
                mtx_unlock(&xbb->lock);
                return (1);
        }

        /*
         * Allocate a reqlist if the caller doesn't have one already.
         */
        if (*reqlist == NULL) {
                nreqlist = xbb_get_reqlist(xbb);
                if (nreqlist == NULL)
                        goto bailout_error;
        }

        /* We always allocate a request. */
        nreq = xbb_get_req(xbb);
        if (nreq == NULL)
                goto bailout_error;

        mtx_unlock(&xbb->lock);

        if (*reqlist == NULL) {
                *reqlist = nreqlist;
                nreqlist->operation = ring_req->operation;
                nreqlist->starting_sector_number = ring_req->sector_number;
                STAILQ_INSERT_TAIL(&xbb->reqlist_pending_stailq, nreqlist,
                                   links);
        }

        nreq->reqlist = *reqlist;
        nreq->req_ring_idx = ring_idx;

        if (xbb->abi != BLKIF_PROTOCOL_NATIVE) {
                bcopy(ring_req, &nreq->ring_req_storage, sizeof(*ring_req));
                nreq->ring_req = &nreq->ring_req_storage;
        } else {
                nreq->ring_req = ring_req;
        }

        binuptime(&nreq->ds_t0);
        devstat_start_transaction(xbb->xbb_stats_in, &nreq->ds_t0);
        STAILQ_INSERT_TAIL(&(*reqlist)->contig_req_list, nreq, links);
        (*reqlist)->num_children++;
        (*reqlist)->nr_segments += ring_req->nr_segments;

        return (0);

bailout_error:

        /*
         * We're out of resources, so set the shortage flag.  The next time
         * a request is released, we'll try waking up the work thread to
         * see if we can allocate more resources.
         */
        xbb->flags |= XBBF_RESOURCE_SHORTAGE;
        xbb->request_shortages++;

        if (nreq != NULL)
                xbb_release_req(xbb, nreq);

        mtx_unlock(&xbb->lock);

        if (nreqlist != NULL)
                xbb_release_reqlist(xbb, nreqlist, /*wakeup*/ 0);

        return (1);
}

/**
 * Create and transmit a response to a blkif request.
 * 
 * \param xbb     Per-instance xbb configuration structure.
 * \param req     The request structure to which to respond.
 * \param status  The status code to report.  See BLKIF_RSP_*
 *                in sys/xen/interface/io/blkif.h.
 */
static void
xbb_send_response(struct xbb_softc *xbb, struct xbb_xen_req *req, int status)
{
        blkif_response_t *resp;
        int               more_to_do;
        int               notify;

        more_to_do = 0;

        /*
         * Place on the response ring for the relevant domain.
         * For now, only the spacing between entries is different
         * in the different ABIs, not the response entry layout.
         */
        mtx_lock(&xbb->lock);
        switch (xbb->abi) {
        case BLKIF_PROTOCOL_NATIVE:
                resp = RING_GET_RESPONSE(&xbb->rings.native,
                                         xbb->rings.native.rsp_prod_pvt);
                break;
        case BLKIF_PROTOCOL_X86_32:
                resp = (blkif_response_t *)
                    RING_GET_RESPONSE(&xbb->rings.x86_32,
                                      xbb->rings.x86_32.rsp_prod_pvt);
                break;
        case BLKIF_PROTOCOL_X86_64:
                resp = (blkif_response_t *)
                    RING_GET_RESPONSE(&xbb->rings.x86_64,
                                      xbb->rings.x86_64.rsp_prod_pvt);
                break;
        default:
                panic("Unexpected blkif protocol ABI.");
        }

        resp->id        = req->id;
        resp->operation = req->operation;
        resp->status    = status;

        xbb->rings.common.rsp_prod_pvt += BLKIF_SEGS_TO_BLOCKS(req->nr_pages);
        RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&xbb->rings.common, notify);

        if (xbb->rings.common.rsp_prod_pvt == xbb->rings.common.req_cons) {

                /*
                 * Tail check for pending requests. Allows frontend to avoid
                 * notifications if requests are already in flight (lower
                 * overheads and promotes batching).
                 */
                RING_FINAL_CHECK_FOR_REQUESTS(&xbb->rings.common, more_to_do);
        } else if (RING_HAS_UNCONSUMED_REQUESTS(&xbb->rings.common)) {

                more_to_do = 1;
        }

        xbb->reqs_completed++;

        mtx_unlock(&xbb->lock);

        if (more_to_do)
                taskqueue_enqueue(xbb->io_taskqueue, &xbb->io_task); 

        if (notify)
                notify_remote_via_irq(xbb->irq);
}

/**
 * Complete a request list.
 *
 * \param xbb        Per-instance xbb configuration structure.
 * \param reqlist    Allocated internal request list structure.
 */
static void
xbb_complete_reqlist(struct xbb_softc *xbb, struct xbb_xen_reqlist *reqlist)
{
        struct xbb_xen_req *nreq;
        off_t               sectors_sent;

        sectors_sent = 0;

        if (reqlist->flags & XBB_REQLIST_MAPPED)
                xbb_unmap_reqlist(reqlist);

        /*
         * All I/O is done, send the response.  A lock should not be
         * necessary here because the request list is complete, and
         * therefore this is the only context accessing this request
         * right now.  The functions we call do their own locking if
         * necessary.
         */
        STAILQ_FOREACH(nreq, &reqlist->contig_req_list, links) {
                off_t cur_sectors_sent;

                xbb_send_response(xbb, nreq, reqlist->status);

                /* We don't report bytes sent if there is an error. */
                if (reqlist->status == BLKIF_RSP_OKAY)
                        cur_sectors_sent = nreq->nr_512b_sectors;
                else
                        cur_sectors_sent = 0;

                sectors_sent += cur_sectors_sent;

                devstat_end_transaction(xbb->xbb_stats_in,
                                        /*bytes*/cur_sectors_sent << 9,
                                        reqlist->ds_tag_type,
                                        reqlist->ds_trans_type,
                                        /*now*/NULL,
                                        /*then*/&nreq->ds_t0);
        }

        /*
         * Take out any sectors not sent.  If we wind up negative (which
         * might happen if an error is reported as well as a residual), just
         * report 0 sectors sent.
         */
        sectors_sent -= reqlist->residual_512b_sectors;
        if (sectors_sent < 0)
                sectors_sent = 0;

        devstat_end_transaction(xbb->xbb_stats,
                                /*bytes*/ sectors_sent << 9,
                                reqlist->ds_tag_type,
                                reqlist->ds_trans_type,
                                /*now*/NULL,
                                /*then*/&reqlist->ds_t0);

        xbb_release_reqlist(xbb, reqlist, /*wakeup*/ 1);
}

/**
 * Completion handler for buffer I/O requests issued by the device
 * backend driver.
 *
 * \param bio  The buffer I/O request on which to perform completion
 *             processing.
 */
static void
xbb_bio_done(struct bio *bio)
{
        struct xbb_softc       *xbb;
        struct xbb_xen_reqlist *reqlist;

        reqlist = bio->bio_caller1;
        xbb     = reqlist->xbb;

        reqlist->residual_512b_sectors += bio->bio_resid >> 9;

        /*
         * This is a bit imprecise.  With aggregated I/O a single
         * request list can contain multiple front-end requests and
         * a multiple bios may point to a single request.  By carefully
         * walking the request list, we could map residuals and errors
         * back to the original front-end request, but the interface
         * isn't sufficiently rich for us to properly report the error.
         * So, we just treat the entire request list as having failed if an
         * error occurs on any part.  And, if an error occurs, we treat
         * the amount of data transferred as 0.
         *
         * For residuals, we report it on the overall aggregated device,
         * but not on the individual requests, since we don't currently
         * do the work to determine which front-end request to which the
         * residual applies.
         */
        if (bio->bio_error) {
                DPRINTF("BIO returned error %d for operation on device %s\n",
                        bio->bio_error, xbb->dev_name);
                reqlist->status = BLKIF_RSP_ERROR;

                if (bio->bio_error == ENXIO
                 && xenbus_get_state(xbb->dev) == XenbusStateConnected) {

                        /*
                         * Backend device has disappeared.  Signal the
                         * front-end that we (the device proxy) want to
                         * go away.
                         */
                        xenbus_set_state(xbb->dev, XenbusStateClosing);
                }
        }

#ifdef XBB_USE_BOUNCE_BUFFERS
        if (bio->bio_cmd == BIO_READ) {
                vm_offset_t kva_offset;

                kva_offset = (vm_offset_t)bio->bio_data
                           - (vm_offset_t)reqlist->bounce;
                memcpy((uint8_t *)reqlist->kva + kva_offset,
                       bio->bio_data, bio->bio_bcount);
        }
#endif /* XBB_USE_BOUNCE_BUFFERS */

        /*
         * Decrement the pending count for the request list.  When we're
         * done with the requests, send status back for all of them.
         */
        if (atomic_fetchadd_int(&reqlist->pendcnt, -1) == 1)
                xbb_complete_reqlist(xbb, reqlist);

        g_destroy_bio(bio);
}

/**
 * Parse a blkif request into an internal request structure and send
 * it to the backend for processing.
 *
 * \param xbb       Per-instance xbb configuration structure.
 * \param reqlist   Allocated internal request list structure.
 *
 * \return          On success, 0.  For resource shortages, non-zero.
 *  
 * This routine performs the backend common aspects of request parsing
 * including compiling an internal request structure, parsing the S/G
 * list and any secondary ring requests in which they may reside, and
 * the mapping of front-end I/O pages into our domain.
 */
static int
xbb_dispatch_io(struct xbb_softc *xbb, struct xbb_xen_reqlist *reqlist)
{
        struct xbb_sg                *xbb_sg;
        struct gnttab_map_grant_ref  *map;
        struct blkif_request_segment *sg;
        struct blkif_request_segment *last_block_sg;
        struct xbb_xen_req           *nreq;
        u_int                         nseg;
        u_int                         seg_idx;
        u_int                         block_segs;
        int                           nr_sects;
        int                           total_sects;
        int                           operation;
        uint8_t                       bio_flags;
        int                           error;

        reqlist->ds_tag_type = DEVSTAT_TAG_SIMPLE;
        bio_flags            = 0;
        total_sects          = 0;
        nr_sects             = 0;

        /*
         * First determine whether we have enough free KVA to satisfy this
         * request list.  If not, tell xbb_run_queue() so it can go to
         * sleep until we have more KVA.
         */
        reqlist->kva = NULL;
        if (reqlist->nr_segments != 0) {
                reqlist->kva = xbb_get_kva(xbb, reqlist->nr_segments);
                if (reqlist->kva == NULL) {
                        /*
                         * If we're out of KVA, return ENOMEM.
                         */
                        return (ENOMEM);
                }
        }

        binuptime(&reqlist->ds_t0);
        devstat_start_transaction(xbb->xbb_stats, &reqlist->ds_t0);

        switch (reqlist->operation) {
        case BLKIF_OP_WRITE_BARRIER:
                bio_flags       |= BIO_ORDERED;
                reqlist->ds_tag_type = DEVSTAT_TAG_ORDERED;
                /* FALLTHROUGH */
        case BLKIF_OP_WRITE:
                operation = BIO_WRITE;
                reqlist->ds_trans_type = DEVSTAT_WRITE;
                if ((xbb->flags & XBBF_READ_ONLY) != 0) {
                        DPRINTF("Attempt to write to read only device %s\n",
                                xbb->dev_name);
                        reqlist->status = BLKIF_RSP_ERROR;
                        goto send_response;
                }
                break;
        case BLKIF_OP_READ:
                operation = BIO_READ;
                reqlist->ds_trans_type = DEVSTAT_READ;
                break;
        case BLKIF_OP_FLUSH_DISKCACHE:
                /*
                 * If this is true, the user has requested that we disable
                 * flush support.  So we just complete the requests
                 * successfully.
                 */
                if (xbb->disable_flush != 0) {
                        goto send_response;
                }

                /*
                 * The user has requested that we only send a real flush
                 * for every N flush requests.  So keep count, and either
                 * complete the request immediately or queue it for the
                 * backend.
                 */
                if (xbb->flush_interval != 0) {
                        if (++(xbb->flush_count) < xbb->flush_interval) {
                                goto send_response;
                        } else
                                xbb->flush_count = 0;
                }

                operation = BIO_FLUSH;
                reqlist->ds_tag_type = DEVSTAT_TAG_ORDERED;
                reqlist->ds_trans_type = DEVSTAT_NO_DATA;
                goto do_dispatch;
                /*NOTREACHED*/
        default:
                DPRINTF("error: unknown block io operation [%d]\n",
                        reqlist->operation);
                reqlist->status = BLKIF_RSP_ERROR;
                goto send_response;
        }

        reqlist->xbb  = xbb;
        xbb_sg        = xbb->xbb_sgs;
        map           = xbb->maps;
        seg_idx       = 0;

        STAILQ_FOREACH(nreq, &reqlist->contig_req_list, links) {
                blkif_request_t         *ring_req;
                RING_IDX                 req_ring_idx;
                u_int                    req_seg_idx;

                ring_req              = nreq->ring_req;
                req_ring_idx          = nreq->req_ring_idx;
                nr_sects              = 0;
                nseg                  = ring_req->nr_segments;
                nreq->id              = ring_req->id;
                nreq->nr_pages        = nseg;
                nreq->nr_512b_sectors = 0;
                req_seg_idx           = 0;
                sg                    = NULL;

                /* Check that number of segments is sane. */
                if (unlikely(nseg == 0)
                 || unlikely(nseg > xbb->max_request_segments)) {
                        DPRINTF("Bad number of segments in request (%d)\n",
                                nseg);
                        reqlist->status = BLKIF_RSP_ERROR;
                        goto send_response;
                }

                block_segs    = MIN(nreq->nr_pages,
                                    BLKIF_MAX_SEGMENTS_PER_HEADER_BLOCK);
                sg            = ring_req->seg;
                last_block_sg = sg + block_segs;
                while (1) {

                        while (sg < last_block_sg) {
                                KASSERT(seg_idx <
                                        XBB_MAX_SEGMENTS_PER_REQLIST,
                                        ("seg_idx %d is too large, max "
                                        "segs %d\n", seg_idx,
                                        XBB_MAX_SEGMENTS_PER_REQLIST));
                        
                                xbb_sg->first_sect = sg->first_sect;
                                xbb_sg->last_sect  = sg->last_sect;
                                xbb_sg->nsect =
                                    (int8_t)(sg->last_sect -
                                    sg->first_sect + 1);

                                if ((sg->last_sect >= (PAGE_SIZE >> 9))
                                 || (xbb_sg->nsect <= 0)) {
                                        reqlist->status = BLKIF_RSP_ERROR;
                                        goto send_response;
                                }

                                nr_sects += xbb_sg->nsect;
                                map->host_addr = xbb_get_gntaddr(reqlist,
                                                        seg_idx, /*sector*/0);
                                KASSERT(map->host_addr + PAGE_SIZE <=
                                        xbb->ring_config.gnt_addr,
                                        ("Host address %#jx len %d overlaps "
                                         "ring address %#jx\n",
                                        (uintmax_t)map->host_addr, PAGE_SIZE,
                                        (uintmax_t)xbb->ring_config.gnt_addr));
                                        
                                map->flags     = GNTMAP_host_map;
                                map->ref       = sg->gref;
                                map->dom       = xbb->otherend_id;
                                if (operation == BIO_WRITE)
                                        map->flags |= GNTMAP_readonly;
                                sg++;
                                map++;
                                xbb_sg++;
                                seg_idx++;
                                req_seg_idx++;
                        }

                        block_segs = MIN(nseg - req_seg_idx,
                                         BLKIF_MAX_SEGMENTS_PER_SEGMENT_BLOCK);
                        if (block_segs == 0)
                                break;

                        /*
                         * Fetch the next request block full of SG elements.
                         * For now, only the spacing between entries is
                         * different in the different ABIs, not the sg entry
                         * layout.
                         */
                        req_ring_idx++;
                        switch (xbb->abi) {
                        case BLKIF_PROTOCOL_NATIVE:
                                sg = BLKRING_GET_SEG_BLOCK(&xbb->rings.native,
                                                           req_ring_idx);
                                break;
                        case BLKIF_PROTOCOL_X86_32:
                        {
                                sg = BLKRING_GET_SEG_BLOCK(&xbb->rings.x86_32,
                                                           req_ring_idx);
                                break;
                        }
                        case BLKIF_PROTOCOL_X86_64:
                        {
                                sg = BLKRING_GET_SEG_BLOCK(&xbb->rings.x86_64,
                                                           req_ring_idx);
                                break;
                        }
                        default:
                                panic("Unexpected blkif protocol ABI.");
                                /* NOTREACHED */
                        } 
                        last_block_sg = sg + block_segs;
                }

                /* Convert to the disk's sector size */
                nreq->nr_512b_sectors = nr_sects;
                nr_sects = (nr_sects << 9) >> xbb->sector_size_shift;
                total_sects += nr_sects;

                if ((nreq->nr_512b_sectors &
                    ((xbb->sector_size >> 9) - 1)) != 0) {
                        device_printf(xbb->dev, "%s: I/O size (%d) is not "
                                      "a multiple of the backing store sector "
                                      "size (%d)\n", __func__,
                                      nreq->nr_512b_sectors << 9,
                                      xbb->sector_size);
                        reqlist->status = BLKIF_RSP_ERROR;
                        goto send_response;
                }
        }

        error = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,
                                          xbb->maps, reqlist->nr_segments);
        if (error != 0)
                panic("Grant table operation failed (%d)", error);

        reqlist->flags |= XBB_REQLIST_MAPPED;

        for (seg_idx = 0, map = xbb->maps; seg_idx < reqlist->nr_segments;
             seg_idx++, map++){

                if (unlikely(map->status != 0)) {
                        DPRINTF("invalid buffer -- could not remap "
                                "it (%d)\n", map->status);
                        DPRINTF("Mapping(%d): Host Addr 0x%lx, flags "
                                "0x%x ref 0x%x, dom %d\n", seg_idx,
                                map->host_addr, map->flags, map->ref,
                                map->dom);
                        reqlist->status = BLKIF_RSP_ERROR;
                        goto send_response;
                }

                reqlist->gnt_handles[seg_idx] = map->handle;
        }
        if (reqlist->starting_sector_number + total_sects >
            xbb->media_num_sectors) {

                DPRINTF("%s of [%" PRIu64 ",%" PRIu64 "] "
                        "extends past end of device %s\n",
                        operation == BIO_READ ? "read" : "write",
                        reqlist->starting_sector_number,
                        reqlist->starting_sector_number + total_sects,
                        xbb->dev_name); 
                reqlist->status = BLKIF_RSP_ERROR;
                goto send_response;
        }

do_dispatch:

        error = xbb->dispatch_io(xbb,
                                 reqlist,
                                 operation,
                                 bio_flags);

        if (error != 0) {
                reqlist->status = BLKIF_RSP_ERROR;
                goto send_response;
        }

        return (0);

send_response:

        xbb_complete_reqlist(xbb, reqlist);

        return (0);
}

static __inline int
xbb_count_sects(blkif_request_t *ring_req)
{
        int i;
        int cur_size = 0;

        for (i = 0; i < ring_req->nr_segments; i++) {
                int nsect;

                nsect = (int8_t)(ring_req->seg[i].last_sect -
                        ring_req->seg[i].first_sect + 1);
                if (nsect <= 0)
                        break;

                cur_size += nsect;
        }

        return (cur_size);
}

/**
 * Process incoming requests from the shared communication ring in response
 * to a signal on the ring's event channel.
 *
 * \param context  Callback argument registerd during task initialization -
 *                 the xbb_softc for this instance.
 * \param pending  The number of taskqueue_enqueue events that have
 *                 occurred since this handler was last run.
 */
static void
xbb_run_queue(void *context, int pending)
{
        struct xbb_softc       *xbb;
        blkif_back_rings_t     *rings;
        RING_IDX                rp;
        uint64_t                cur_sector;
        int                     cur_operation;
        struct xbb_xen_reqlist *reqlist;


        xbb   = (struct xbb_softc *)context;
        rings = &xbb->rings;

        /*
         * Work gather and dispatch loop.  Note that we have a bias here
         * towards gathering I/O sent by blockfront.  We first gather up
         * everything in the ring, as long as we have resources.  Then we
         * dispatch one request, and then attempt to gather up any
         * additional requests that have come in while we were dispatching
         * the request.
         *
         * This allows us to get a clearer picture (via devstat) of how
         * many requests blockfront is queueing to us at any given time.
         */
        for (;;) {
                int retval;

                /*
                 * Initialize reqlist to the last element in the pending
                 * queue, if there is one.  This allows us to add more
                 * requests to that request list, if we have room.
                 */
                reqlist = STAILQ_LAST(&xbb->reqlist_pending_stailq,
                                      xbb_xen_reqlist, links);
                if (reqlist != NULL) {
                        cur_sector = reqlist->next_contig_sector;
                        cur_operation = reqlist->operation;
                } else {
                        cur_operation = 0;
                        cur_sector    = 0;
                }

                /*
                 * Cache req_prod to avoid accessing a cache line shared
                 * with the frontend.
                 */
                rp = rings->common.sring->req_prod;

                /* Ensure we see queued requests up to 'rp'. */
                rmb();

                /**
                 * Run so long as there is work to consume and the generation
                 * of a response will not overflow the ring.
                 *
                 * @note There's a 1 to 1 relationship between requests and
                 *       responses, so an overflow should never occur.  This
                 *       test is to protect our domain from digesting bogus
                 *       data.  Shouldn't we log this?
                 */
                while (rings->common.req_cons != rp
                    && RING_REQUEST_CONS_OVERFLOW(&rings->common,
                                                  rings->common.req_cons) == 0){
                        blkif_request_t         ring_req_storage;
                        blkif_request_t        *ring_req;
                        int                     cur_size;

                        switch (xbb->abi) {
                        case BLKIF_PROTOCOL_NATIVE:
                                ring_req = RING_GET_REQUEST(&xbb->rings.native,
                                    rings->common.req_cons);
                                break;
                        case BLKIF_PROTOCOL_X86_32:
                        {
                                struct blkif_x86_32_request *ring_req32;

                                ring_req32 = RING_GET_REQUEST(
                                    &xbb->rings.x86_32, rings->common.req_cons);
                                blkif_get_x86_32_req(&ring_req_storage,
                                                     ring_req32);
                                ring_req = &ring_req_storage;
                                break;
                        }
                        case BLKIF_PROTOCOL_X86_64:
                        {
                                struct blkif_x86_64_request *ring_req64;

                                ring_req64 =RING_GET_REQUEST(&xbb->rings.x86_64,
                                    rings->common.req_cons);
                                blkif_get_x86_64_req(&ring_req_storage,
                                                     ring_req64);
                                ring_req = &ring_req_storage;
                                break;
                        }
                        default:
                                panic("Unexpected blkif protocol ABI.");
                                /* NOTREACHED */
                        } 

                        /*
                         * Check for situations that would require closing
                         * off this I/O for further coalescing:
                         *  - Coalescing is turned off.
                         *  - Current I/O is out of sequence with the previous
                         *    I/O.
                         *  - Coalesced I/O would be too large.
                         */
                        if ((reqlist != NULL)
                         && ((xbb->no_coalesce_reqs != 0)
                          || ((xbb->no_coalesce_reqs == 0)
                           && ((ring_req->sector_number != cur_sector)
                            || (ring_req->operation != cur_operation)
                            || ((ring_req->nr_segments + reqlist->nr_segments) >
                                 xbb->max_reqlist_segments))))) {
                                reqlist = NULL;
                        }

                        /*
                         * Grab and check for all resources in one shot.
                         * If we can't get all of the resources we need,
                         * the shortage is noted and the thread will get
                         * woken up when more resources are available.
                         */
                        retval = xbb_get_resources(xbb, &reqlist, ring_req,
                                                   xbb->rings.common.req_cons);

                        if (retval != 0) {
                                /*
                                 * Resource shortage has been recorded.
                                 * We'll be scheduled to run once a request
                                 * object frees up due to a completion.
                                 */
                                break;
                        }

                        /*
                         * Signify that we can overwrite this request with
                         * a response by incrementing our consumer index.
                         * The response won't be generated until after
                         * we've already consumed all necessary data out
                         * of the version of the request in the ring buffer
                         * (for native mode).  We must update the consumer
                         * index  before issueing back-end I/O so there is
                         * no possibility that it will complete and a
                         * response be generated before we make room in 
                         * the queue for that response.
                         */
                        xbb->rings.common.req_cons +=
                            BLKIF_SEGS_TO_BLOCKS(ring_req->nr_segments);
                        xbb->reqs_received++;

                        cur_size = xbb_count_sects(ring_req);
                        cur_sector = ring_req->sector_number + cur_size;
                        reqlist->next_contig_sector = cur_sector;
                        cur_operation = ring_req->operation;
                }

                /* Check for I/O to dispatch */
                reqlist = STAILQ_FIRST(&xbb->reqlist_pending_stailq);
                if (reqlist == NULL) {
                        /*
                         * We're out of work to do, put the task queue to
                         * sleep.
                         */
                        break;
                }

                /*
                 * Grab the first request off the queue and attempt
                 * to dispatch it.
                 */
                STAILQ_REMOVE_HEAD(&xbb->reqlist_pending_stailq, links);

                retval = xbb_dispatch_io(xbb, reqlist);
                if (retval != 0) {
                        /*
                         * xbb_dispatch_io() returns non-zero only when
                         * there is a resource shortage.  If that's the
                         * case, re-queue this request on the head of the
                         * queue, and go to sleep until we have more
                         * resources.
                         */
                        STAILQ_INSERT_HEAD(&xbb->reqlist_pending_stailq,
                                           reqlist, links);
                        break;
                } else {
                        /*
                         * If we still have anything on the queue after
                         * removing the head entry, that is because we
                         * met one of the criteria to create a new
                         * request list (outlined above), and we'll call
                         * that a forced dispatch for statistical purposes.
                         *
                         * Otherwise, if there is only one element on the
                         * queue, we coalesced everything available on
                         * the ring and we'll call that a normal dispatch.
                         */
                        reqlist = STAILQ_FIRST(&xbb->reqlist_pending_stailq);

                        if (reqlist != NULL)
                                xbb->forced_dispatch++;
                        else
                                xbb->normal_dispatch++;

                        xbb->total_dispatch++;
                }
        }
}

/**
 * Interrupt handler bound to the shared ring's event channel.
 *
 * \param arg  Callback argument registerd during event channel
 *             binding - the xbb_softc for this instance.
 */
static void
xbb_intr(void *arg)
{
        struct xbb_softc *xbb;

        /* Defer to kernel thread. */
        xbb = (struct xbb_softc *)arg;
        taskqueue_enqueue(xbb->io_taskqueue, &xbb->io_task); 
}

SDT_PROVIDER_DEFINE(xbb);
SDT_PROBE_DEFINE1(xbb, kernel, xbb_dispatch_dev, flush, flush, "int");
SDT_PROBE_DEFINE3(xbb, kernel, xbb_dispatch_dev, read, read, "int", "uint64_t",
                  "uint64_t");
SDT_PROBE_DEFINE3(xbb, kernel, xbb_dispatch_dev, write, write, "int",
                  "uint64_t", "uint64_t");

/*----------------------------- Backend Handlers -----------------------------*/
/**
 * Backend handler for character device access.
 *
 * \param xbb        Per-instance xbb configuration structure.
 * \param reqlist    Allocated internal request list structure.
 * \param operation  BIO_* I/O operation code.
 * \param bio_flags  Additional bio_flag data to pass to any generated
 *                   bios (e.g. BIO_ORDERED)..
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_dispatch_dev(struct xbb_softc *xbb, struct xbb_xen_reqlist *reqlist,
                 int operation, int bio_flags)
{
        struct xbb_dev_data *dev_data;
        struct bio          *bios[XBB_MAX_SEGMENTS_PER_REQLIST];
        struct xbb_xen_req  *nreq;
        off_t                bio_offset;
        struct bio          *bio;
        struct xbb_sg       *xbb_sg;
        u_int                nbio;
        u_int                bio_idx;
        u_int                nseg;
        u_int                seg_idx;
        int                  error;

        dev_data   = &xbb->backend.dev;
        bio_offset = (off_t)reqlist->starting_sector_number
                   << xbb->sector_size_shift;
        error      = 0;
        nbio       = 0;
        bio_idx    = 0;

        if (operation == BIO_FLUSH) {
                nreq = STAILQ_FIRST(&reqlist->contig_req_list);
                bio = g_new_bio();
                if (unlikely(bio == NULL)) {
                        DPRINTF("Unable to allocate bio for BIO_FLUSH\n");
                        error = ENOMEM;
                        return (error);
                }

                bio->bio_cmd     = BIO_FLUSH;
                bio->bio_flags  |= BIO_ORDERED;
                bio->bio_dev     = dev_data->cdev;
                bio->bio_offset  = 0;
                bio->bio_data    = 0;
                bio->bio_done    = xbb_bio_done;
                bio->bio_caller1 = nreq;
                bio->bio_pblkno  = 0;

                nreq->pendcnt    = 1;

                SDT_PROBE1(xbb, kernel, xbb_dispatch_dev, flush,
                           device_get_unit(xbb->dev));

                (*dev_data->csw->d_strategy)(bio);

                return (0);
        }

        xbb_sg = xbb->xbb_sgs;
        bio    = NULL;
        nseg = reqlist->nr_segments;

        for (seg_idx = 0; seg_idx < nseg; seg_idx++, xbb_sg++) {

                /*
                 * KVA will not be contiguous, so any additional
                 * I/O will need to be represented in a new bio.
                 */
                if ((bio != NULL)
                 && (xbb_sg->first_sect != 0)) {
                        if ((bio->bio_length & (xbb->sector_size - 1)) != 0) {
                                printf("%s: Discontiguous I/O request "
                                       "from domain %d ends on "
                                       "non-sector boundary\n",
                                       __func__, xbb->otherend_id);
                                error = EINVAL;
                                goto fail_free_bios;
                        }
                        bio = NULL;
                }

                if (bio == NULL) {
                        /*
                         * Make sure that the start of this bio is
                         * aligned to a device sector.
                         */
                        if ((bio_offset & (xbb->sector_size - 1)) != 0){
                                printf("%s: Misaligned I/O request "
                                       "from domain %d\n", __func__,
                                       xbb->otherend_id);
                                error = EINVAL;
                                goto fail_free_bios;
                        }

                        bio = bios[nbio++] = g_new_bio();
                        if (unlikely(bio == NULL)) {
                                error = ENOMEM;
                                goto fail_free_bios;
                        }
                        bio->bio_cmd     = operation;
                        bio->bio_flags  |= bio_flags;
                        bio->bio_dev     = dev_data->cdev;
                        bio->bio_offset  = bio_offset;
                        bio->bio_data    = xbb_reqlist_ioaddr(reqlist, seg_idx,
                                                xbb_sg->first_sect);
                        bio->bio_done    = xbb_bio_done;
                        bio->bio_caller1 = reqlist;
                        bio->bio_pblkno  = bio_offset >> xbb->sector_size_shift;
                }

                bio->bio_length += xbb_sg->nsect << 9;
                bio->bio_bcount  = bio->bio_length;
                bio_offset      += xbb_sg->nsect << 9;

                if (xbb_sg->last_sect != (PAGE_SIZE - 512) >> 9) {

                        if ((bio->bio_length & (xbb->sector_size - 1)) != 0) {
                                printf("%s: Discontiguous I/O request "
                                       "from domain %d ends on "
                                       "non-sector boundary\n",
                                       __func__, xbb->otherend_id);
                                error = EINVAL;
                                goto fail_free_bios;
                        }
                        /*
                         * KVA will not be contiguous, so any additional
                         * I/O will need to be represented in a new bio.
                         */
                        bio = NULL;
                }
        }

        reqlist->pendcnt = nbio;

        for (bio_idx = 0; bio_idx < nbio; bio_idx++)
        {
#ifdef XBB_USE_BOUNCE_BUFFERS
                vm_offset_t kva_offset;

                kva_offset = (vm_offset_t)bios[bio_idx]->bio_data
                           - (vm_offset_t)reqlist->bounce;
                if (operation == BIO_WRITE) {
                        memcpy(bios[bio_idx]->bio_data,
                               (uint8_t *)reqlist->kva + kva_offset,
                               bios[bio_idx]->bio_bcount);
                }
#endif
                if (operation == BIO_READ) {
                        SDT_PROBE3(xbb, kernel, xbb_dispatch_dev, read,
                                   device_get_unit(xbb->dev),
                                   bios[bio_idx]->bio_offset,
                                   bios[bio_idx]->bio_length);
                } else if (operation == BIO_WRITE) {
                        SDT_PROBE3(xbb, kernel, xbb_dispatch_dev, write,
                                   device_get_unit(xbb->dev),
                                   bios[bio_idx]->bio_offset,
                                   bios[bio_idx]->bio_length);
                }
                (*dev_data->csw->d_strategy)(bios[bio_idx]);
        }

        return (error);

fail_free_bios:
        for (bio_idx = 0; bio_idx < (nbio-1); bio_idx++)
                g_destroy_bio(bios[bio_idx]);
        
        return (error);
}

SDT_PROBE_DEFINE1(xbb, kernel, xbb_dispatch_file, flush, flush, "int");
SDT_PROBE_DEFINE3(xbb, kernel, xbb_dispatch_file, read, read, "int", "uint64_t",
                  "uint64_t");
SDT_PROBE_DEFINE3(xbb, kernel, xbb_dispatch_file, write, write, "int",
                  "uint64_t", "uint64_t");

/**
 * Backend handler for file access.
 *
 * \param xbb        Per-instance xbb configuration structure.
 * \param reqlist    Allocated internal request list.
 * \param operation  BIO_* I/O operation code.
 * \param flags      Additional bio_flag data to pass to any generated bios
 *                   (e.g. BIO_ORDERED)..
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_dispatch_file(struct xbb_softc *xbb, struct xbb_xen_reqlist *reqlist,
                  int operation, int flags)
{
        struct xbb_file_data *file_data;
        u_int                 seg_idx;
        u_int                 nseg;
        off_t                 sectors_sent;
        struct uio            xuio;
        struct xbb_sg        *xbb_sg;
        struct iovec         *xiovec;
#ifdef XBB_USE_BOUNCE_BUFFERS
        void                **p_vaddr;
        int                   saved_uio_iovcnt;
#endif /* XBB_USE_BOUNCE_BUFFERS */
        int                   vfs_is_locked;
        int                   error;

        file_data = &xbb->backend.file;
        sectors_sent = 0;
        error = 0;
        bzero(&xuio, sizeof(xuio));

        switch (operation) {
        case BIO_READ:
                xuio.uio_rw = UIO_READ;
                break;
        case BIO_WRITE:
                xuio.uio_rw = UIO_WRITE;
                break;
        case BIO_FLUSH: {
                struct mount *mountpoint;

                SDT_PROBE1(xbb, kernel, xbb_dispatch_file, flush,
                           device_get_unit(xbb->dev));

                vfs_is_locked = VFS_LOCK_GIANT(xbb->vn->v_mount);

                (void) vn_start_write(xbb->vn, &mountpoint, V_WAIT);

                vn_lock(xbb->vn, LK_EXCLUSIVE | LK_RETRY);
                error = VOP_FSYNC(xbb->vn, MNT_WAIT, curthread);
                VOP_UNLOCK(xbb->vn, 0);

                vn_finished_write(mountpoint);

                VFS_UNLOCK_GIANT(vfs_is_locked);

                goto bailout_send_response;
                /* NOTREACHED */
        }
        default:
                panic("invalid operation %d", operation);
                /* NOTREACHED */
        }
        xuio.uio_offset = (vm_offset_t)reqlist->starting_sector_number
                        << xbb->sector_size_shift;
        xuio.uio_segflg = UIO_SYSSPACE;
        xuio.uio_iov = file_data->xiovecs;
        xuio.uio_iovcnt = 0;
        xbb_sg = xbb->xbb_sgs;
        nseg = reqlist->nr_segments;

        for (xiovec = NULL, seg_idx = 0; seg_idx < nseg; seg_idx++, xbb_sg++) {

                /*
                 * If the first sector is not 0, the KVA will
                 * not be contiguous and we'll need to go on
                 * to another segment.
                 */
                if (xbb_sg->first_sect != 0)
                        xiovec = NULL;

                if (xiovec == NULL) {
                        xiovec = &file_data->xiovecs[xuio.uio_iovcnt];
                        xiovec->iov_base = xbb_reqlist_ioaddr(reqlist,
                            seg_idx, xbb_sg->first_sect);
#ifdef XBB_USE_BOUNCE_BUFFERS
                        /*
                         * Store the address of the incoming
                         * buffer at this particular offset
                         * as well, so we can do the copy
                         * later without having to do more
                         * work to recalculate this address.
                         */
                        p_vaddr = &file_data->xiovecs_vaddr[xuio.uio_iovcnt];
                        *p_vaddr = xbb_reqlist_vaddr(reqlist, seg_idx,
                            xbb_sg->first_sect);
#endif /* XBB_USE_BOUNCE_BUFFERS */
                        xiovec->iov_len = 0;
                        xuio.uio_iovcnt++;
                }

                xiovec->iov_len += xbb_sg->nsect << 9;

                xuio.uio_resid += xbb_sg->nsect << 9;

                /*
                 * If the last sector is not the full page
                 * size count, the next segment will not be
                 * contiguous in KVA and we need a new iovec.
                 */
                if (xbb_sg->last_sect != (PAGE_SIZE - 512) >> 9)
                        xiovec = NULL;
        }

        xuio.uio_td = curthread;

#ifdef XBB_USE_BOUNCE_BUFFERS
        saved_uio_iovcnt = xuio.uio_iovcnt;

        if (operation == BIO_WRITE) {
                /* Copy the write data to the local buffer. */
                for (seg_idx = 0, p_vaddr = file_data->xiovecs_vaddr,
                     xiovec = xuio.uio_iov; seg_idx < xuio.uio_iovcnt;
                     seg_idx++, xiovec++, p_vaddr++) {

                        memcpy(xiovec->iov_base, *p_vaddr, xiovec->iov_len);
                }
        } else {
                /*
                 * We only need to save off the iovecs in the case of a
                 * read, because the copy for the read happens after the
                 * VOP_READ().  (The uio will get modified in that call
                 * sequence.)
                 */
                memcpy(file_data->saved_xiovecs, xuio.uio_iov,
                       xuio.uio_iovcnt * sizeof(xuio.uio_iov[0]));
        }
#endif /* XBB_USE_BOUNCE_BUFFERS */

        vfs_is_locked = VFS_LOCK_GIANT(xbb->vn->v_mount);
        switch (operation) {
        case BIO_READ:

                SDT_PROBE3(xbb, kernel, xbb_dispatch_file, read,
                           device_get_unit(xbb->dev), xuio.uio_offset,
                           xuio.uio_resid);

                vn_lock(xbb->vn, LK_EXCLUSIVE | LK_RETRY);

                /*
                 * UFS pays attention to IO_DIRECT for reads.  If the
                 * DIRECTIO option is configured into the kernel, it calls
                 * ffs_rawread().  But that only works for single-segment
                 * uios with user space addresses.  In our case, with a
                 * kernel uio, it still reads into the buffer cache, but it
                 * will just try to release the buffer from the cache later
                 * on in ffs_read().
                 *
                 * ZFS does not pay attention to IO_DIRECT for reads.
                 *
                 * UFS does not pay attention to IO_SYNC for reads.
                 *
                 * ZFS pays attention to IO_SYNC (which translates into the
                 * Solaris define FRSYNC for zfs_read()) for reads.  It
                 * attempts to sync the file before reading.
                 *
                 * So, to attempt to provide some barrier semantics in the
                 * BIO_ORDERED case, set both IO_DIRECT and IO_SYNC.  
                 */
                error = VOP_READ(xbb->vn, &xuio, (flags & BIO_ORDERED) ? 
                                 (IO_DIRECT|IO_SYNC) : 0, file_data->cred);

                VOP_UNLOCK(xbb->vn, 0);
                break;
        case BIO_WRITE: {
                struct mount *mountpoint;

                SDT_PROBE3(xbb, kernel, xbb_dispatch_file, write,
                           device_get_unit(xbb->dev), xuio.uio_offset,
                           xuio.uio_resid);

                (void)vn_start_write(xbb->vn, &mountpoint, V_WAIT);

                vn_lock(xbb->vn, LK_EXCLUSIVE | LK_RETRY);

                /*
                 * UFS pays attention to IO_DIRECT for writes.  The write
                 * is done asynchronously.  (Normally the write would just
                 * get put into cache.
                 *
                 * UFS pays attention to IO_SYNC for writes.  It will
                 * attempt to write the buffer out synchronously if that
                 * flag is set.
                 *
                 * ZFS does not pay attention to IO_DIRECT for writes.
                 *
                 * ZFS pays attention to IO_SYNC (a.k.a. FSYNC or FRSYNC)
                 * for writes.  It will flush the transaction from the
                 * cache before returning.
                 *
                 * So if we've got the BIO_ORDERED flag set, we want
                 * IO_SYNC in either the UFS or ZFS case.
                 */
                error = VOP_WRITE(xbb->vn, &xuio, (flags & BIO_ORDERED) ?
                                  IO_SYNC : 0, file_data->cred);
                VOP_UNLOCK(xbb->vn, 0);

                vn_finished_write(mountpoint);

                break;
        }
        default:
                panic("invalid operation %d", operation);
                /* NOTREACHED */
        }
        VFS_UNLOCK_GIANT(vfs_is_locked);

#ifdef XBB_USE_BOUNCE_BUFFERS
        /* We only need to copy here for read operations */
        if (operation == BIO_READ) {

                for (seg_idx = 0, p_vaddr = file_data->xiovecs_vaddr,
                     xiovec = file_data->saved_xiovecs;
                     seg_idx < saved_uio_iovcnt; seg_idx++,
                     xiovec++, p_vaddr++) {

                        /*
                         * Note that we have to use the copy of the 
                         * io vector we made above.  uiomove() modifies
                         * the uio and its referenced vector as uiomove
                         * performs the copy, so we can't rely on any
                         * state from the original uio.
                         */
                        memcpy(*p_vaddr, xiovec->iov_base, xiovec->iov_len);
                }
        }
#endif /* XBB_USE_BOUNCE_BUFFERS */

bailout_send_response:

        if (error != 0)
                reqlist->status = BLKIF_RSP_ERROR;

        xbb_complete_reqlist(xbb, reqlist);

        return (0);
}

/*--------------------------- Backend Configuration --------------------------*/
/**
 * Close and cleanup any backend device/file specific state for this
 * block back instance. 
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static void
xbb_close_backend(struct xbb_softc *xbb)
{
        DROP_GIANT();
        DPRINTF("closing dev=%s\n", xbb->dev_name);
        if (xbb->vn) {
                int flags = FREAD;
                int vfs_is_locked = 0;

                if ((xbb->flags & XBBF_READ_ONLY) == 0)
                        flags |= FWRITE;

                switch (xbb->device_type) {
                case XBB_TYPE_DISK:
                        if (xbb->backend.dev.csw) {
                                dev_relthread(xbb->backend.dev.cdev,
                                              xbb->backend.dev.dev_ref);
                                xbb->backend.dev.csw  = NULL;
                                xbb->backend.dev.cdev = NULL;
                        }
                        break;
                case XBB_TYPE_FILE:
                        vfs_is_locked = VFS_LOCK_GIANT(xbb->vn->v_mount);
                        break;
                case XBB_TYPE_NONE:
                default:
                        panic("Unexpected backend type.");
                        break;
                }

                (void)vn_close(xbb->vn, flags, NOCRED, curthread);
                xbb->vn = NULL;

                switch (xbb->device_type) {
                case XBB_TYPE_DISK:
                        break;
                case XBB_TYPE_FILE:
                        VFS_UNLOCK_GIANT(vfs_is_locked);
                        if (xbb->backend.file.cred != NULL) {
                                crfree(xbb->backend.file.cred);
                                xbb->backend.file.cred = NULL;
                        }
                        break;
                case XBB_TYPE_NONE:
                default:
                        panic("Unexpected backend type.");
                        break;
                }
        }
        PICKUP_GIANT();
}

/**
 * Open a character device to be used for backend I/O.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_open_dev(struct xbb_softc *xbb)
{
        struct vattr   vattr;
        struct cdev   *dev;
        struct cdevsw *devsw;
        int            error;

        xbb->device_type = XBB_TYPE_DISK;
        xbb->dispatch_io = xbb_dispatch_dev;
        xbb->backend.dev.cdev = xbb->vn->v_rdev;
        xbb->backend.dev.csw = dev_refthread(xbb->backend.dev.cdev,
                                             &xbb->backend.dev.dev_ref);
        if (xbb->backend.dev.csw == NULL)
                panic("Unable to retrieve device switch");

        error = VOP_GETATTR(xbb->vn, &vattr, NOCRED);
        if (error) {
                xenbus_dev_fatal(xbb->dev, error, "error getting "
                                 "vnode attributes for device %s",
                                 xbb->dev_name);
                return (error);
        }


        dev = xbb->vn->v_rdev;
        devsw = dev->si_devsw;
        if (!devsw->d_ioctl) {
                xenbus_dev_fatal(xbb->dev, ENODEV, "no d_ioctl for "
                                 "device %s!", xbb->dev_name);
                return (ENODEV);
        }

        error = devsw->d_ioctl(dev, DIOCGSECTORSIZE,
                               (caddr_t)&xbb->sector_size, FREAD,
                               curthread);
        if (error) {
                xenbus_dev_fatal(xbb->dev, error,
                                 "error calling ioctl DIOCGSECTORSIZE "
                                 "for device %s", xbb->dev_name);
                return (error);
        }

        error = devsw->d_ioctl(dev, DIOCGMEDIASIZE,
                               (caddr_t)&xbb->media_size, FREAD,
                               curthread);
        if (error) {
                xenbus_dev_fatal(xbb->dev, error,
                                 "error calling ioctl DIOCGMEDIASIZE "
                                 "for device %s", xbb->dev_name);
                return (error);
        }

        return (0);
}

/**
 * Open a file to be used for backend I/O.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_open_file(struct xbb_softc *xbb)
{
        struct xbb_file_data *file_data;
        struct vattr          vattr;
        int                   error;

        file_data = &xbb->backend.file;
        xbb->device_type = XBB_TYPE_FILE;
        xbb->dispatch_io = xbb_dispatch_file;
        error = VOP_GETATTR(xbb->vn, &vattr, curthread->td_ucred);
        if (error != 0) {
                xenbus_dev_fatal(xbb->dev, error,
                                 "error calling VOP_GETATTR()"
                                 "for file %s", xbb->dev_name);
                return (error);
        }

        /*
         * Verify that we have the ability to upgrade to exclusive
         * access on this file so we can trap errors at open instead
         * of reporting them during first access.
         */
        if (VOP_ISLOCKED(xbb->vn) != LK_EXCLUSIVE) {
                vn_lock(xbb->vn, LK_UPGRADE | LK_RETRY);
                if (xbb->vn->v_iflag & VI_DOOMED) {
                        error = EBADF;
                        xenbus_dev_fatal(xbb->dev, error,
                                         "error locking file %s",
                                         xbb->dev_name);

                        return (error);
                }
        }

        file_data->cred = crhold(curthread->td_ucred);
        xbb->media_size = vattr.va_size;

        /*
         * XXX KDM vattr.va_blocksize may be larger than 512 bytes here.
         * With ZFS, it is 131072 bytes.  Block sizes that large don't work
         * with disklabel and UFS on FreeBSD at least.  Large block sizes
         * may not work with other OSes as well.  So just export a sector
         * size of 512 bytes, which should work with any OS or
         * application.  Since our backing is a file, any block size will
         * work fine for the backing store.
         */
#if 0
        xbb->sector_size = vattr.va_blocksize;
#endif
        xbb->sector_size = 512;

        /*
         * Sanity check.  The media size has to be at least one
         * sector long.
         */
        if (xbb->media_size < xbb->sector_size) {
                error = EINVAL;
                xenbus_dev_fatal(xbb->dev, error,
                                 "file %s size %ju < block size %u",
                                 xbb->dev_name,
                                 (uintmax_t)xbb->media_size,
                                 xbb->sector_size);
        }
        return (error);
}

/**
 * Open the backend provider for this connection.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_open_backend(struct xbb_softc *xbb)
{
        struct nameidata nd;
        int              flags;
        int              error;
        int              vfs_is_locked;

        flags = FREAD;
        error = 0;

        DPRINTF("opening dev=%s\n", xbb->dev_name);

        if (rootvnode == NULL) {
                xenbus_dev_fatal(xbb->dev, ENOENT,
                                 "Root file system not mounted");
                return (ENOENT);
        }

        if ((xbb->flags & XBBF_READ_ONLY) == 0)
                flags |= FWRITE;

        if (!curthread->td_proc->p_fd->fd_cdir) {
                curthread->td_proc->p_fd->fd_cdir = rootvnode;
                VREF(rootvnode);
        }
        if (!curthread->td_proc->p_fd->fd_rdir) {
                curthread->td_proc->p_fd->fd_rdir = rootvnode;
                VREF(rootvnode);
        }
        if (!curthread->td_proc->p_fd->fd_jdir) {
                curthread->td_proc->p_fd->fd_jdir = rootvnode;
                VREF(rootvnode);
        }

 again:
        NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, xbb->dev_name, curthread);
        error = vn_open(&nd, &flags, 0, NULL);
        if (error) {
                /*
                 * This is the only reasonable guess we can make as far as
                 * path if the user doesn't give us a fully qualified path.
                 * If they want to specify a file, they need to specify the
                 * full path.
                 */
                if (xbb->dev_name[0] != '/') {
                        char *dev_path = "/dev/";
                        char *dev_name;

                        /* Try adding device path at beginning of name */
                        dev_name = malloc(strlen(xbb->dev_name)
                                        + strlen(dev_path) + 1,
                                          M_XENBLOCKBACK, M_NOWAIT);
                        if (dev_name) {
                                sprintf(dev_name, "%s%s", dev_path,
                                        xbb->dev_name);
                                free(xbb->dev_name, M_XENBLOCKBACK);
                                xbb->dev_name = dev_name;
                                goto again;
                        }
                }
                xenbus_dev_fatal(xbb->dev, error, "error opening device %s",
                                 xbb->dev_name);
                return (error);
        }

        vfs_is_locked = NDHASGIANT(&nd);

        NDFREE(&nd, NDF_ONLY_PNBUF);
                
        xbb->vn = nd.ni_vp;

        /* We only support disks and files. */
        if (vn_isdisk(xbb->vn, &error)) {
                error = xbb_open_dev(xbb);
        } else if (xbb->vn->v_type == VREG) {
                error = xbb_open_file(xbb);
        } else {
                error = EINVAL;
                xenbus_dev_fatal(xbb->dev, error, "%s is not a disk "
                                 "or file", xbb->dev_name);
        }
        VOP_UNLOCK(xbb->vn, 0);
        VFS_UNLOCK_GIANT(vfs_is_locked);

        if (error != 0) {
                xbb_close_backend(xbb);
                return (error);
        }

        xbb->sector_size_shift = fls(xbb->sector_size) - 1;
        xbb->media_num_sectors = xbb->media_size >> xbb->sector_size_shift;

        DPRINTF("opened %s=%s sector_size=%u media_size=%" PRId64 "\n",
                (xbb->device_type == XBB_TYPE_DISK) ? "dev" : "file",
                xbb->dev_name, xbb->sector_size, xbb->media_size);

        return (0);
}

/*------------------------ Inter-Domain Communication ------------------------*/
/**
 * Free dynamically allocated KVA or pseudo-physical address allocations.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static void
xbb_free_communication_mem(struct xbb_softc *xbb)
{
        if (xbb->kva != 0) {
#ifndef XENHVM
                kmem_free(kernel_map, xbb->kva, xbb->kva_size);
#else
                if (xbb->pseudo_phys_res != NULL) {
                        bus_release_resource(xbb->dev, SYS_RES_MEMORY,
                                             xbb->pseudo_phys_res_id,
                                             xbb->pseudo_phys_res);
                        xbb->pseudo_phys_res = NULL;
                }
#endif
        }
        xbb->kva = 0;
        xbb->gnt_base_addr = 0;
        if (xbb->kva_free != NULL) {
                free(xbb->kva_free, M_XENBLOCKBACK);
                xbb->kva_free = NULL;
        }
}

/**
 * Cleanup all inter-domain communication mechanisms.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static int
xbb_disconnect(struct xbb_softc *xbb)
{
        struct gnttab_unmap_grant_ref  ops[XBB_MAX_RING_PAGES];
        struct gnttab_unmap_grant_ref *op;
        u_int                          ring_idx;
        int                            error;

        DPRINTF("\n");

        if ((xbb->flags & XBBF_RING_CONNECTED) == 0)
                return (0);

        if (xbb->irq != 0) {
                unbind_from_irqhandler(xbb->irq);
                xbb->irq = 0;
        }

        mtx_unlock(&xbb->lock);
        taskqueue_drain(xbb->io_taskqueue, &xbb->io_task); 
        mtx_lock(&xbb->lock);

        /*
         * No new interrupts can generate work, but we must wait
         * for all currently active requests to drain.
         */
        if (xbb->active_request_count != 0)
                return (EAGAIN);
        
        for (ring_idx = 0, op = ops;
             ring_idx < xbb->ring_config.ring_pages;
             ring_idx++, op++) {

                op->host_addr    = xbb->ring_config.gnt_addr
                                 + (ring_idx * PAGE_SIZE);
                op->dev_bus_addr = xbb->ring_config.bus_addr[ring_idx];
                op->handle       = xbb->ring_config.handle[ring_idx];
        }

        error = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, ops,
                                          xbb->ring_config.ring_pages);
        if (error != 0)
                panic("Grant table op failed (%d)", error);

        xbb_free_communication_mem(xbb);

        if (xbb->requests != NULL) {
                free(xbb->requests, M_XENBLOCKBACK);
                xbb->requests = NULL;
        }

        if (xbb->request_lists != NULL) {
                struct xbb_xen_reqlist *reqlist;
                int i;

                /* There is one request list for ever allocated request. */
                for (i = 0, reqlist = xbb->request_lists;
                     i < xbb->max_requests; i++, reqlist++){
#ifdef XBB_USE_BOUNCE_BUFFERS
                        if (reqlist->bounce != NULL) {
                                free(reqlist->bounce, M_XENBLOCKBACK);
                                reqlist->bounce = NULL;
                        }
#endif
                        if (reqlist->gnt_handles != NULL) {
                                free(reqlist->gnt_handles, M_XENBLOCKBACK);
                                reqlist->gnt_handles = NULL;
                        }
                }
                free(xbb->request_lists, M_XENBLOCKBACK);
                xbb->request_lists = NULL;
        }

        xbb->flags &= ~XBBF_RING_CONNECTED;
        return (0);
}

/**
 * Map shared memory ring into domain local address space, initialize
 * ring control structures, and bind an interrupt to the event channel
 * used to notify us of ring changes.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static int
xbb_connect_ring(struct xbb_softc *xbb)
{
        struct gnttab_map_grant_ref  gnts[XBB_MAX_RING_PAGES];
        struct gnttab_map_grant_ref *gnt;
        u_int                        ring_idx;
        int                          error;

        if ((xbb->flags & XBBF_RING_CONNECTED) != 0)
                return (0);

        /*
         * Kva for our ring is at the tail of the region of kva allocated
         * by xbb_alloc_communication_mem().
         */
        xbb->ring_config.va = xbb->kva
                            + (xbb->kva_size
                             - (xbb->ring_config.ring_pages * PAGE_SIZE));
        xbb->ring_config.gnt_addr = xbb->gnt_base_addr
                                  + (xbb->kva_size
                                   - (xbb->ring_config.ring_pages * PAGE_SIZE));

        for (ring_idx = 0, gnt = gnts;
             ring_idx < xbb->ring_config.ring_pages;
             ring_idx++, gnt++) {

                gnt->host_addr = xbb->ring_config.gnt_addr
                               + (ring_idx * PAGE_SIZE);
                gnt->flags     = GNTMAP_host_map;
                gnt->ref       = xbb->ring_config.ring_ref[ring_idx];
                gnt->dom       = xbb->otherend_id;
        }

        error = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, gnts,
                                          xbb->ring_config.ring_pages);
        if (error)
                panic("blkback: Ring page grant table op failed (%d)", error);

        for (ring_idx = 0, gnt = gnts;
             ring_idx < xbb->ring_config.ring_pages;
             ring_idx++, gnt++) {
                if (gnt->status != 0) {
                        xbb->ring_config.va = 0;
                        xenbus_dev_fatal(xbb->dev, EACCES,
                                         "Ring shared page mapping failed. "
                                         "Status %d.", gnt->status);
                        return (EACCES);
                }
                xbb->ring_config.handle[ring_idx]   = gnt->handle;
                xbb->ring_config.bus_addr[ring_idx] = gnt->dev_bus_addr;
        }

        /* Initialize the ring based on ABI. */
        switch (xbb->abi) {
        case BLKIF_PROTOCOL_NATIVE:
        {
                blkif_sring_t *sring;
                sring = (blkif_sring_t *)xbb->ring_config.va;
                BACK_RING_INIT(&xbb->rings.native, sring,
                               xbb->ring_config.ring_pages * PAGE_SIZE);
                break;
        }
        case BLKIF_PROTOCOL_X86_32:
        {
                blkif_x86_32_sring_t *sring_x86_32;
                sring_x86_32 = (blkif_x86_32_sring_t *)xbb->ring_config.va;
                BACK_RING_INIT(&xbb->rings.x86_32, sring_x86_32,
                               xbb->ring_config.ring_pages * PAGE_SIZE);
                break;
        }
        case BLKIF_PROTOCOL_X86_64:
        {
                blkif_x86_64_sring_t *sring_x86_64;
                sring_x86_64 = (blkif_x86_64_sring_t *)xbb->ring_config.va;
                BACK_RING_INIT(&xbb->rings.x86_64, sring_x86_64,
                               xbb->ring_config.ring_pages * PAGE_SIZE);
                break;
        }
        default:
                panic("Unexpected blkif protocol ABI.");
        }

        xbb->flags |= XBBF_RING_CONNECTED;

        error =
            bind_interdomain_evtchn_to_irqhandler(xbb->otherend_id,
                                                  xbb->ring_config.evtchn,
                                                  device_get_nameunit(xbb->dev),
                                                  xbb_intr, /*arg*/xbb,
                                                  INTR_TYPE_BIO | INTR_MPSAFE,
                                                  &xbb->irq);
        if (error) {
                (void)xbb_disconnect(xbb);
                xenbus_dev_fatal(xbb->dev, error, "binding event channel");
                return (error);
        }

        DPRINTF("rings connected!\n");

        return 0;
}

/* Needed to make bit_alloc() macro work */
#define calloc(count, size) malloc((count)*(size), M_XENBLOCKBACK,      \
                                   M_NOWAIT|M_ZERO);

/**
 * Size KVA and pseudo-physical address allocations based on negotiated
 * values for the size and number of I/O requests, and the size of our
 * communication ring.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * These address spaces are used to dynamically map pages in the
 * front-end's domain into our own.
 */
static int
xbb_alloc_communication_mem(struct xbb_softc *xbb)
{
        xbb->reqlist_kva_pages = xbb->max_requests * xbb->max_request_segments;
        xbb->reqlist_kva_size = xbb->reqlist_kva_pages * PAGE_SIZE;
        xbb->kva_size = xbb->reqlist_kva_size +
                        (xbb->ring_config.ring_pages * PAGE_SIZE);

        xbb->kva_free = bit_alloc(xbb->reqlist_kva_pages);
        if (xbb->kva_free == NULL)
                return (ENOMEM);

        DPRINTF("%s: kva_size = %d, reqlist_kva_size = %d\n",
                device_get_nameunit(xbb->dev), xbb->kva_size,
                xbb->reqlist_kva_size);
#ifndef XENHVM
        xbb->kva = kmem_alloc_nofault(kernel_map, xbb->kva_size);
        if (xbb->kva == 0)
                return (ENOMEM);
        xbb->gnt_base_addr = xbb->kva;
#else /* XENHVM */
        /*
         * Reserve a range of pseudo physical memory that we can map
         * into kva.  These pages will only be backed by machine
         * pages ("real memory") during the lifetime of front-end requests
         * via grant table operations.
         */
        xbb->pseudo_phys_res_id = 0;
        xbb->pseudo_phys_res = bus_alloc_resource(xbb->dev, SYS_RES_MEMORY,
                                                  &xbb->pseudo_phys_res_id,
                                                  0, ~0, xbb->kva_size,
                                                  RF_ACTIVE);
        if (xbb->pseudo_phys_res == NULL) {
                xbb->kva = 0;
                return (ENOMEM);
        }
        xbb->kva = (vm_offset_t)rman_get_virtual(xbb->pseudo_phys_res);
        xbb->gnt_base_addr = rman_get_start(xbb->pseudo_phys_res);
#endif /* XENHVM */

        DPRINTF("%s: kva: %#jx, gnt_base_addr: %#jx\n",
                device_get_nameunit(xbb->dev), (uintmax_t)xbb->kva,
                (uintmax_t)xbb->gnt_base_addr); 
        return (0);
}

/**
 * Collect front-end information from the XenStore.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static int
xbb_collect_frontend_info(struct xbb_softc *xbb)
{
        char        protocol_abi[64];
        const char *otherend_path;
        int         error;
        u_int       ring_idx;
        u_int       ring_page_order;
        size_t      ring_size;

        otherend_path = xenbus_get_otherend_path(xbb->dev);

        /*
         * Protocol defaults valid even if all negotiation fails.
         */
        xbb->ring_config.ring_pages = 1;
        xbb->max_request_segments   = BLKIF_MAX_SEGMENTS_PER_HEADER_BLOCK;
        xbb->max_request_size       = xbb->max_request_segments * PAGE_SIZE;

        /*
         * Mandatory data (used in all versions of the protocol) first.
         */
        error = xs_scanf(XST_NIL, otherend_path,
                         "event-channel", NULL, "%" PRIu32,
                         &xbb->ring_config.evtchn);
        if (error != 0) {
                xenbus_dev_fatal(xbb->dev, error,
                                 "Unable to retrieve event-channel information "
                                 "from frontend %s.  Unable to connect.",
                                 xenbus_get_otherend_path(xbb->dev));
                return (error);
        }

        /*
         * These fields are initialized to legacy protocol defaults
         * so we only need to fail if reading the updated value succeeds
         * and the new value is outside of its allowed range.
         *
         * \note xs_gather() returns on the first encountered error, so
         *       we must use independant calls in order to guarantee
         *       we don't miss information in a sparsly populated front-end
         *       tree.
         *
         * \note xs_scanf() does not update variables for unmatched
         *       fields.
         */
        ring_page_order = 0;
        (void)xs_scanf(XST_NIL, otherend_path,
                       "ring-page-order", NULL, "%u",
                       &ring_page_order);
        xbb->ring_config.ring_pages = 1 << ring_page_order;
        (void)xs_scanf(XST_NIL, otherend_path,
                       "num-ring-pages", NULL, "%u",
                       &xbb->ring_config.ring_pages);
        ring_size = PAGE_SIZE * xbb->ring_config.ring_pages;
        xbb->max_requests = BLKIF_MAX_RING_REQUESTS(ring_size);

        (void)xs_scanf(XST_NIL, otherend_path,
                       "max-requests", NULL, "%u",
                       &xbb->max_requests);

        (void)xs_scanf(XST_NIL, otherend_path,
                       "max-request-segments", NULL, "%u",
                       &xbb->max_request_segments);

        (void)xs_scanf(XST_NIL, otherend_path,
                       "max-request-size", NULL, "%u",
                       &xbb->max_request_size);

        if (xbb->ring_config.ring_pages > XBB_MAX_RING_PAGES) {
                xenbus_dev_fatal(xbb->dev, EINVAL,
                                 "Front-end specified ring-pages of %u "
                                 "exceeds backend limit of %zu.  "
                                 "Unable to connect.",
                                 xbb->ring_config.ring_pages,
                                 XBB_MAX_RING_PAGES);
                return (EINVAL);
        } else if (xbb->max_requests > XBB_MAX_REQUESTS) {
                xenbus_dev_fatal(xbb->dev, EINVAL,
                                 "Front-end specified max_requests of %u "
                                 "exceeds backend limit of %u.  "
                                 "Unable to connect.",
                                 xbb->max_requests,
                                 XBB_MAX_REQUESTS);
                return (EINVAL);
        } else if (xbb->max_request_segments > XBB_MAX_SEGMENTS_PER_REQUEST) {
                xenbus_dev_fatal(xbb->dev, EINVAL,
                                 "Front-end specified max_requests_segments "
                                 "of %u exceeds backend limit of %u.  "
                                 "Unable to connect.",
                                 xbb->max_request_segments,
                                 XBB_MAX_SEGMENTS_PER_REQUEST);
                return (EINVAL);
        } else if (xbb->max_request_size > XBB_MAX_REQUEST_SIZE) {
                xenbus_dev_fatal(xbb->dev, EINVAL,
                                 "Front-end specified max_request_size "
                                 "of %u exceeds backend limit of %u.  "
                                 "Unable to connect.",
                                 xbb->max_request_size,
                                 XBB_MAX_REQUEST_SIZE);
                return (EINVAL);
        }

        if (xbb->ring_config.ring_pages == 1) {
                error = xs_gather(XST_NIL, otherend_path,
                                  "ring-ref", "%" PRIu32,
                                  &xbb->ring_config.ring_ref[0],
                                  NULL);
                if (error != 0) {
                        xenbus_dev_fatal(xbb->dev, error,
                                         "Unable to retrieve ring information "
                                         "from frontend %s.  Unable to "
                                         "connect.",
                                         xenbus_get_otherend_path(xbb->dev));
                        return (error);
                }
        } else {
                /* Multi-page ring format. */
                for (ring_idx = 0; ring_idx < xbb->ring_config.ring_pages;
                     ring_idx++) {
                        char ring_ref_name[]= "ring_refXX";

                        snprintf(ring_ref_name, sizeof(ring_ref_name),
                                 "ring-ref%u", ring_idx);
                        error = xs_scanf(XST_NIL, otherend_path,
                                         ring_ref_name, NULL, "%" PRIu32,
                                         &xbb->ring_config.ring_ref[ring_idx]);
                        if (error != 0) {
                                xenbus_dev_fatal(xbb->dev, error,
                                                 "Failed to retriev grant "
                                                 "reference for page %u of "
                                                 "shared ring.  Unable "
                                                 "to connect.", ring_idx);
                                return (error);
                        }
                }
        }

        error = xs_gather(XST_NIL, otherend_path,
                          "protocol", "%63s", protocol_abi,
                          NULL); 
        if (error != 0
         || !strcmp(protocol_abi, XEN_IO_PROTO_ABI_NATIVE)) {
                /*
                 * Assume native if the frontend has not
                 * published ABI data or it has published and
                 * matches our own ABI.
                 */
                xbb->abi = BLKIF_PROTOCOL_NATIVE;
        } else if (!strcmp(protocol_abi, XEN_IO_PROTO_ABI_X86_32)) {

                xbb->abi = BLKIF_PROTOCOL_X86_32;
        } else if (!strcmp(protocol_abi, XEN_IO_PROTO_ABI_X86_64)) {

                xbb->abi = BLKIF_PROTOCOL_X86_64;
        } else {

                xenbus_dev_fatal(xbb->dev, EINVAL,
                                 "Unknown protocol ABI (%s) published by "
                                 "frontend.  Unable to connect.", protocol_abi);
                return (EINVAL);
        }
        return (0);
}

/**
 * Allocate per-request data structures given request size and number
 * information negotiated with the front-end.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static int
xbb_alloc_requests(struct xbb_softc *xbb)
{
        struct xbb_xen_req *req;
        struct xbb_xen_req *last_req;

        /*
         * Allocate request book keeping datastructures.
         */
        xbb->requests = malloc(xbb->max_requests * sizeof(*xbb->requests),
                               M_XENBLOCKBACK, M_NOWAIT|M_ZERO);
        if (xbb->requests == NULL) {
                xenbus_dev_fatal(xbb->dev, ENOMEM, 
                                  "Unable to allocate request structures");
                return (ENOMEM);
        }

        req      = xbb->requests;
        last_req = &xbb->requests[xbb->max_requests - 1];
        STAILQ_INIT(&xbb->request_free_stailq);
        while (req <= last_req) {
                STAILQ_INSERT_TAIL(&xbb->request_free_stailq, req, links);
                req++;
        }
        return (0);
}

static int
xbb_alloc_request_lists(struct xbb_softc *xbb)
{
        struct xbb_xen_reqlist *reqlist;
        int                     i;

        /*
         * If no requests can be merged, we need 1 request list per
         * in flight request.
         */
        xbb->request_lists = malloc(xbb->max_requests *
                sizeof(*xbb->request_lists), M_XENBLOCKBACK, M_NOWAIT|M_ZERO);
        if (xbb->request_lists == NULL) {
                xenbus_dev_fatal(xbb->dev, ENOMEM, 
                                  "Unable to allocate request list structures");
                return (ENOMEM);
        }

        STAILQ_INIT(&xbb->reqlist_free_stailq);
        STAILQ_INIT(&xbb->reqlist_pending_stailq);
        for (i = 0; i < xbb->max_requests; i++) {
                int seg;

                reqlist      = &xbb->request_lists[i];

                reqlist->xbb = xbb;

#ifdef XBB_USE_BOUNCE_BUFFERS
                reqlist->bounce = malloc(xbb->max_reqlist_size,
                                         M_XENBLOCKBACK, M_NOWAIT);
                if (reqlist->bounce == NULL) {
                        xenbus_dev_fatal(xbb->dev, ENOMEM, 
                                         "Unable to allocate request "
                                         "bounce buffers");
                        return (ENOMEM);
                }
#endif /* XBB_USE_BOUNCE_BUFFERS */

                reqlist->gnt_handles = malloc(xbb->max_reqlist_segments *
                                              sizeof(*reqlist->gnt_handles),
                                              M_XENBLOCKBACK, M_NOWAIT|M_ZERO);
                if (reqlist->gnt_handles == NULL) {
                        xenbus_dev_fatal(xbb->dev, ENOMEM,
                                          "Unable to allocate request "
                                          "grant references");
                        return (ENOMEM);
                }

                for (seg = 0; seg < xbb->max_reqlist_segments; seg++)
                        reqlist->gnt_handles[seg] = GRANT_REF_INVALID;

                STAILQ_INSERT_TAIL(&xbb->reqlist_free_stailq, reqlist, links);
        }
        return (0);
}

/**
 * Supply information about the physical device to the frontend
 * via XenBus.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static int
xbb_publish_backend_info(struct xbb_softc *xbb)
{
        struct xs_transaction xst;
        const char           *our_path;
        const char           *leaf;
        int                   error;

        our_path = xenbus_get_node(xbb->dev);
        while (1) {
                error = xs_transaction_start(&xst);
                if (error != 0) {
                        xenbus_dev_fatal(xbb->dev, error,
                                         "Error publishing backend info "
                                         "(start transaction)");
                        return (error);
                }

                leaf = "sectors";
                error = xs_printf(xst, our_path, leaf,
                                  "%"PRIu64, xbb->media_num_sectors);
                if (error != 0)
                        break;

                /* XXX Support all VBD attributes here. */
                leaf = "info";
                error = xs_printf(xst, our_path, leaf, "%u",
                                  xbb->flags & XBBF_READ_ONLY
                                ? VDISK_READONLY : 0);
                if (error != 0)
                        break;

                leaf = "sector-size";
                error = xs_printf(xst, our_path, leaf, "%u",
                                  xbb->sector_size);
                if (error != 0)
                        break;

                error = xs_transaction_end(xst, 0);
                if (error == 0) {
                        return (0);
                } else if (error != EAGAIN) {
                        xenbus_dev_fatal(xbb->dev, error, "ending transaction");
                        return (error);
                }
        }

        xenbus_dev_fatal(xbb->dev, error, "writing %s/%s",
                        our_path, leaf);
        xs_transaction_end(xst, 1);
        return (error);
}

/**
 * Connect to our blkfront peer now that it has completed publishing
 * its configuration into the XenStore.
 *
 * \param xbb  Per-instance xbb configuration structure.
 */
static void
xbb_connect(struct xbb_softc *xbb)
{
        int error;

        if (xenbus_get_state(xbb->dev) == XenbusStateConnected)
                return;

        if (xbb_collect_frontend_info(xbb) != 0)
                return;

        xbb->flags &= ~XBBF_SHUTDOWN;

        /*
         * We limit the maximum number of reqlist segments to the maximum
         * number of segments in the ring, or our absolute maximum,
         * whichever is smaller.
         */
        xbb->max_reqlist_segments = MIN(xbb->max_request_segments *
                xbb->max_requests, XBB_MAX_SEGMENTS_PER_REQLIST);

        /*
         * The maximum size is simply a function of the number of segments
         * we can handle.
         */
        xbb->max_reqlist_size = xbb->max_reqlist_segments * PAGE_SIZE;

        /* Allocate resources whose size depends on front-end configuration. */
        error = xbb_alloc_communication_mem(xbb);
        if (error != 0) {
                xenbus_dev_fatal(xbb->dev, error,
                                 "Unable to allocate communication memory");
                return;
        }

        error = xbb_alloc_requests(xbb);
        if (error != 0) {
                /* Specific errors are reported by xbb_alloc_requests(). */
                return;
        }

        error = xbb_alloc_request_lists(xbb);
        if (error != 0) {
                /* Specific errors are reported by xbb_alloc_request_lists(). */
                return;
        }

        /*
         * Connect communication channel.
         */
        error = xbb_connect_ring(xbb);
        if (error != 0) {
                /* Specific errors are reported by xbb_connect_ring(). */
                return;
        }
        
        if (xbb_publish_backend_info(xbb) != 0) {
                /*
                 * If we can't publish our data, we cannot participate
                 * in this connection, and waiting for a front-end state
                 * change will not help the situation.
                 */
                (void)xbb_disconnect(xbb);
                return;
        }

        /* Ready for I/O. */
        xenbus_set_state(xbb->dev, XenbusStateConnected);
}

/*-------------------------- Device Teardown Support -------------------------*/
/**
 * Perform device shutdown functions.
 *
 * \param xbb  Per-instance xbb configuration structure.
 *
 * Mark this instance as shutting down, wait for any active I/O on the
 * backend device/file to drain, disconnect from the front-end, and notify
 * any waiters (e.g. a thread invoking our detach method) that detach can
 * now proceed.
 */
static int
xbb_shutdown(struct xbb_softc *xbb)
{
        XenbusState frontState;
        int         error;

        DPRINTF("\n");

        /*
         * Due to the need to drop our mutex during some
         * xenbus operations, it is possible for two threads
         * to attempt to close out shutdown processing at
         * the same time.  Tell the caller that hits this
         * race to try back later. 
         */
        if ((xbb->flags & XBBF_IN_SHUTDOWN) != 0)
                return (EAGAIN);

        xbb->flags |= XBBF_IN_SHUTDOWN;
        mtx_unlock(&xbb->lock);

        if (xenbus_get_state(xbb->dev) < XenbusStateClosing)
                xenbus_set_state(xbb->dev, XenbusStateClosing);

        frontState = xenbus_get_otherend_state(xbb->dev);
        mtx_lock(&xbb->lock);
        xbb->flags &= ~XBBF_IN_SHUTDOWN;

        /* The front can submit I/O until entering the closed state. */
        if (frontState < XenbusStateClosed)
                return (EAGAIN);

        DPRINTF("\n");

        /* Indicate shutdown is in progress. */
        xbb->flags |= XBBF_SHUTDOWN;

        /* Disconnect from the front-end. */
        error = xbb_disconnect(xbb);
        if (error != 0) {
                /*
                 * Requests still outstanding.  We'll be called again
                 * once they complete.
                 */
                KASSERT(error == EAGAIN,
                        ("%s: Unexpected xbb_disconnect() failure %d",
                         __func__, error));

                return (error);
        }

        DPRINTF("\n");

        /* Indicate to xbb_detach() that is it safe to proceed. */
        wakeup(xbb);

        return (0);
}

/**
 * Report an attach time error to the console and Xen, and cleanup
 * this instance by forcing immediate detach processing.
 *
 * \param xbb  Per-instance xbb configuration structure.
 * \param err  Errno describing the error.
 * \param fmt  Printf style format and arguments
 */
static void
xbb_attach_failed(struct xbb_softc *xbb, int err, const char *fmt, ...)
{
        va_list ap;
        va_list ap_hotplug;

        va_start(ap, fmt);
        va_copy(ap_hotplug, ap);
        xs_vprintf(XST_NIL, xenbus_get_node(xbb->dev),
                  "hotplug-error", fmt, ap_hotplug);
        va_end(ap_hotplug);
        xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                  "hotplug-status", "error");

        xenbus_dev_vfatal(xbb->dev, err, fmt, ap);
        va_end(ap);

        xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                  "online", "0");
        xbb_detach(xbb->dev);
}

/*---------------------------- NewBus Entrypoints ----------------------------*/
/**
 * Inspect a XenBus device and claim it if is of the appropriate type.
 * 
 * \param dev  NewBus device object representing a candidate XenBus device.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_probe(device_t dev)
{
 
        if (!strcmp(xenbus_get_type(dev), "vbd")) {
                device_set_desc(dev, "Backend Virtual Block Device");
                device_quiet(dev);
                return (0);
        }

        return (ENXIO);
}

/**
 * Setup sysctl variables to control various Block Back parameters.
 *
 * \param xbb  Xen Block Back softc.
 *
 */
static void
xbb_setup_sysctl(struct xbb_softc *xbb)
{
        struct sysctl_ctx_list *sysctl_ctx = NULL;
        struct sysctl_oid      *sysctl_tree = NULL;
        
        sysctl_ctx = device_get_sysctl_ctx(xbb->dev);
        if (sysctl_ctx == NULL)
                return;

        sysctl_tree = device_get_sysctl_tree(xbb->dev);
        if (sysctl_tree == NULL)
                return;

        SYSCTL_ADD_INT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                       "disable_flush", CTLFLAG_RW, &xbb->disable_flush, 0,
                       "fake the flush command");

        SYSCTL_ADD_INT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                       "flush_interval", CTLFLAG_RW, &xbb->flush_interval, 0,
                       "send a real flush for N flush requests");

        SYSCTL_ADD_INT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                       "no_coalesce_reqs", CTLFLAG_RW, &xbb->no_coalesce_reqs,0,
                       "Don't coalesce contiguous requests");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "reqs_received", CTLFLAG_RW, &xbb->reqs_received,
                         "how many I/O requests we have received");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "reqs_completed", CTLFLAG_RW, &xbb->reqs_completed,
                         "how many I/O requests have been completed");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "forced_dispatch", CTLFLAG_RW, &xbb->forced_dispatch,
                         "how many I/O dispatches were forced");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "normal_dispatch", CTLFLAG_RW, &xbb->normal_dispatch,
                         "how many I/O dispatches were normal");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "total_dispatch", CTLFLAG_RW, &xbb->total_dispatch,
                         "total number of I/O dispatches");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "kva_shortages", CTLFLAG_RW, &xbb->kva_shortages,
                         "how many times we have run out of KVA");

        SYSCTL_ADD_UQUAD(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                         "request_shortages", CTLFLAG_RW,
                         &xbb->request_shortages,
                         "how many times we have run out of requests");

        SYSCTL_ADD_UINT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                        "max_requests", CTLFLAG_RD, &xbb->max_requests, 0,
                        "maximum outstanding requests (negotiated)");

        SYSCTL_ADD_UINT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                        "max_request_segments", CTLFLAG_RD,
                        &xbb->max_request_segments, 0,
                        "maximum number of pages per requests (negotiated)");

        SYSCTL_ADD_UINT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                        "max_request_size", CTLFLAG_RD,
                        &xbb->max_request_size, 0,
                        "maximum size in bytes of a request (negotiated)");

        SYSCTL_ADD_UINT(sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree), OID_AUTO,
                        "ring_pages", CTLFLAG_RD,
                        &xbb->ring_config.ring_pages, 0,
                        "communication channel pages (negotiated)");
}

/**
 * Attach to a XenBus device that has been claimed by our probe routine.
 *
 * \param dev  NewBus device object representing this Xen Block Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_attach(device_t dev)
{
        struct xbb_softc        *xbb;
        int                      error;
        u_int                    max_ring_page_order;

        DPRINTF("Attaching to %s\n", xenbus_get_node(dev));

        /*
         * Basic initialization.
         * After this block it is safe to call xbb_detach()
         * to clean up any allocated data for this instance.
         */
        xbb = device_get_softc(dev);
        xbb->dev = dev;
        xbb->otherend_id = xenbus_get_otherend_id(dev);
        TASK_INIT(&xbb->io_task, /*priority*/0, xbb_run_queue, xbb);
        mtx_init(&xbb->lock, device_get_nameunit(dev), NULL, MTX_DEF);

        /*
         * Publish protocol capabilities for consumption by the
         * front-end.
         */
        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "feature-barrier", "1");
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/feature-barrier",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "feature-flush-cache", "1");
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/feature-flush-cache",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        /*
         * Amazon EC2 client compatility.  They refer to max-ring-pages
         * instead of to max-ring-page-order.
         */
        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "max-ring-pages", "%zu", XBB_MAX_RING_PAGES);
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/max-ring-pages",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        max_ring_page_order = flsl(XBB_MAX_RING_PAGES) - 1;
        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "max-ring-page-order", "%u", max_ring_page_order);
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/max-ring-page-order",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "max-requests", "%u", XBB_MAX_REQUESTS);
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/max-requests",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "max-request-segments", "%u",
                          XBB_MAX_SEGMENTS_PER_REQUEST);
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/max-request-segments",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "max-request-size", "%u",
                          XBB_MAX_REQUEST_SIZE);
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/max-request-size",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        /* Collect physical device information. */
        error = xs_gather(XST_NIL, xenbus_get_otherend_path(xbb->dev),
                          "device-type", NULL, &xbb->dev_type,
                          NULL);
        if (error != 0)
                xbb->dev_type = NULL;

        error = xs_gather(XST_NIL, xenbus_get_node(dev),
                          "mode", NULL, &xbb->dev_mode,
                          "params", NULL, &xbb->dev_name,
                          NULL);
        if (error != 0) {
                xbb_attach_failed(xbb, error, "reading backend fields at %s",
                                  xenbus_get_node(dev));
                return (ENXIO);
        }

        /* Parse fopen style mode flags. */
        if (strchr(xbb->dev_mode, 'w') == NULL)
                xbb->flags |= XBBF_READ_ONLY;

        /*
         * Verify the physical device is present and can support
         * the desired I/O mode.
         */
        DROP_GIANT();
        error = xbb_open_backend(xbb);
        PICKUP_GIANT();
        if (error != 0) {
                xbb_attach_failed(xbb, error, "Unable to open %s",
                                  xbb->dev_name);
                return (ENXIO);
        }

        /* Use devstat(9) for recording statistics. */
        xbb->xbb_stats = devstat_new_entry("xbb", device_get_unit(xbb->dev),
                                           xbb->sector_size,
                                           DEVSTAT_ALL_SUPPORTED,
                                           DEVSTAT_TYPE_DIRECT
                                         | DEVSTAT_TYPE_IF_OTHER,
                                           DEVSTAT_PRIORITY_OTHER);

        xbb->xbb_stats_in = devstat_new_entry("xbbi", device_get_unit(xbb->dev),
                                              xbb->sector_size,
                                              DEVSTAT_ALL_SUPPORTED,
                                              DEVSTAT_TYPE_DIRECT
                                            | DEVSTAT_TYPE_IF_OTHER,
                                              DEVSTAT_PRIORITY_OTHER);
        /*
         * Setup sysctl variables.
         */
        xbb_setup_sysctl(xbb);

        /*
         * Create a taskqueue for doing work that must occur from a
         * thread context.
         */
        xbb->io_taskqueue = taskqueue_create(device_get_nameunit(dev), M_NOWAIT,
                                             taskqueue_thread_enqueue,
                                             /*context*/&xbb->io_taskqueue);
        if (xbb->io_taskqueue == NULL) {
                xbb_attach_failed(xbb, error, "Unable to create taskqueue");
                return (ENOMEM);
        }

        taskqueue_start_threads(&xbb->io_taskqueue,
                                /*num threads*/1,
                                /*priority*/PWAIT,
                                /*thread name*/
                                "%s taskq", device_get_nameunit(dev));

        /* Update hot-plug status to satisfy xend. */
        error = xs_printf(XST_NIL, xenbus_get_node(xbb->dev),
                          "hotplug-status", "connected");
        if (error) {
                xbb_attach_failed(xbb, error, "writing %s/hotplug-status",
                                  xenbus_get_node(xbb->dev));
                return (error);
        }

        /* Tell the front end that we are ready to connect. */
        xenbus_set_state(dev, XenbusStateInitWait);

        return (0);
}

/**
 * Detach from a block back device instance.
 *
 * \param dev  NewBus device object representing this Xen Block Back instance.
 *
 * \return  0 for success, errno codes for failure.
 * 
 * \note A block back device may be detached at any time in its life-cycle,
 *       including part way through the attach process.  For this reason,
 *       initialization order and the intialization state checks in this
 *       routine must be carefully coupled so that attach time failures
 *       are gracefully handled.
 */
static int
xbb_detach(device_t dev)
{
        struct xbb_softc *xbb;

        DPRINTF("\n");

        xbb = device_get_softc(dev);
        mtx_lock(&xbb->lock);
        while (xbb_shutdown(xbb) == EAGAIN) {
                msleep(xbb, &xbb->lock, /*wakeup prio unchanged*/0,
                       "xbb_shutdown", 0);
        }
        mtx_unlock(&xbb->lock);

        DPRINTF("\n");

        if (xbb->io_taskqueue != NULL)
                taskqueue_free(xbb->io_taskqueue);

        if (xbb->xbb_stats != NULL)
                devstat_remove_entry(xbb->xbb_stats);

        if (xbb->xbb_stats_in != NULL)
                devstat_remove_entry(xbb->xbb_stats_in);

        xbb_close_backend(xbb);

        if (xbb->dev_mode != NULL) {
                free(xbb->dev_mode, M_XENBUS);
                xbb->dev_mode = NULL;
        }

        if (xbb->dev_type != NULL) {
                free(xbb->dev_type, M_XENBUS);
                xbb->dev_type = NULL;
        }

        if (xbb->dev_name != NULL) {
                free(xbb->dev_name, M_XENBUS);
                xbb->dev_name = NULL;
        }

        mtx_destroy(&xbb->lock);
        return (0);
}

/**
 * Prepare this block back device for suspension of this VM.
 * 
 * \param dev  NewBus device object representing this Xen Block Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_suspend(device_t dev)
{
#ifdef NOT_YET
        struct xbb_softc *sc = device_get_softc(dev);

        /* Prevent new requests being issued until we fix things up. */
        mtx_lock(&sc->xb_io_lock);
        sc->connected = BLKIF_STATE_SUSPENDED;
        mtx_unlock(&sc->xb_io_lock);
#endif

        return (0);
}

/**
 * Perform any processing required to recover from a suspended state.
 * 
 * \param dev  NewBus device object representing this Xen Block Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xbb_resume(device_t dev)
{
        return (0);
}

/**
 * Handle state changes expressed via the XenStore by our front-end peer.
 *
 * \param dev             NewBus device object representing this Xen
 *                        Block Back instance.
 * \param frontend_state  The new state of the front-end.
 *
 * \return  0 for success, errno codes for failure.
 */
static void
xbb_frontend_changed(device_t dev, XenbusState frontend_state)
{
        struct xbb_softc *xbb = device_get_softc(dev);

        DPRINTF("frontend_state=%s, xbb_state=%s\n",
                xenbus_strstate(frontend_state),
                xenbus_strstate(xenbus_get_state(xbb->dev)));

        switch (frontend_state) {
        case XenbusStateInitialising:
                break;
        case XenbusStateInitialised:
        case XenbusStateConnected:
                xbb_connect(xbb);
                break;
        case XenbusStateClosing:
        case XenbusStateClosed:
                mtx_lock(&xbb->lock);
                xbb_shutdown(xbb);
                mtx_unlock(&xbb->lock);
                if (frontend_state == XenbusStateClosed)
                        xenbus_set_state(xbb->dev, XenbusStateClosed);
                break;
        default:
                xenbus_dev_fatal(xbb->dev, EINVAL, "saw state %d at frontend",
                                 frontend_state);
                break;
        }
}

/*---------------------------- NewBus Registration ---------------------------*/
static device_method_t xbb_methods[] = {
        /* Device interface */
        DEVMETHOD(device_probe,         xbb_probe),
        DEVMETHOD(device_attach,        xbb_attach),
        DEVMETHOD(device_detach,        xbb_detach),
        DEVMETHOD(device_shutdown,      bus_generic_shutdown),
        DEVMETHOD(device_suspend,       xbb_suspend),
        DEVMETHOD(device_resume,        xbb_resume),

        /* Xenbus interface */
        DEVMETHOD(xenbus_otherend_changed, xbb_frontend_changed),

        { 0, 0 }
};

static driver_t xbb_driver = {
        "xbbd",
        xbb_methods,
        sizeof(struct xbb_softc),
};
devclass_t xbb_devclass;

DRIVER_MODULE(xbbd, xenbusb_back, xbb_driver, xbb_devclass, 0, 0);