1 .\" ----------------------------------------------------------------------------
2 .\" "THE BEER-WARE LICENSE" (Revision 42):
3 .\" <phk@FreeBSD.org> wrote this file. As long as you retain this notice you
4 .\" can do whatever you want with this stuff. If we meet some day, and you think
5 .\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
6 .\" ----------------------------------------------------------------------------
17 To compile this driver into the kernel,
18 place the following lines in your
19 kernel configuration file:
20 .Bd -ragged -offset indent
24 Alternatively, to load the driver as a
25 module at boot time, place the following line in
27 .Bd -literal -offset indent
33 driver provides support for four kinds of memory backed virtual disks:
34 .Bl -tag -width preload
36 Backing store is allocated using
38 Only one malloc-bucket is used, which means that all
42 backing must share the malloc-per-bucket-quota.
43 The exact size of this quota varies, in particular with the amount
46 The exact value can be determined with
53 is used for backing store.
54 For backwards compatibility the type
57 See the description of module loading directives in
59 and note that the module name will either be an absolute path to the image file
60 or the name of a file in the
63 If the kernel is created with option
65 the first preloaded image found will become the root file system.
67 A regular file is used as backing store.
68 This allows for mounting ISO images without the tedious
69 detour over actual physical media.
71 Backing store is allocated from buffer memory.
72 Pages get pushed out to the swap when the system is under memory
73 pressure, otherwise they stay in the operating memory.
76 backing is generally preferable over
81 For more information, please see
84 To create a kernel with a ramdisk or MD file system, your kernel config
85 needs the following options:
86 .Bd -literal -offset indent
87 options MD_ROOT # MD is a potential root device
88 options MD_ROOT_READONLY # disallow mounting root writeable
89 options MD_ROOT_SIZE=8192 # 8MB ram disk
90 makeoptions MFS_IMAGE=/h/foo/ARM-MD
91 options ROOTDEVNAME=\\"ufs:md0\\"
96 will be loaded as the initial image each boot.
97 To create the image to use, please follow the steps to create a file-backed
101 Other tools will also create these images, such as NanoBSD.
102 .Sh ARM KERNEL OPTIONS
103 On armv6 and armv7 architectures, an MD_ROOT image larger than
104 approximately 55 MiB may require building a custom kernel using
105 several tuning options related to kernel memory usage.
106 .Bl -tag -width indent
107 .It Cd options LOCORE_MAP_MB=<num>
108 This configures how much memory is mapped for the kernel during
109 the early initialization stages.
110 The value must be at least as large as the kernel plus all preloaded
111 modules, including the root image.
112 There is no downside to setting this value too large, as long
113 as it does not exceed the amount of physical memory.
114 The default is 64 MiB.
115 .It Cd options NKPT2PG=<num>
116 This configures the number of kernel L2 page table pages to
117 preallocate during kernel initialization.
118 Each L2 page can map 4 MiB of kernel space.
119 The value must be large enough to map the kernel plus all preloaded
120 modules, including the root image.
121 The default value is 32, which is sufficient to map 128 MiB.
122 .It Cd options VM_KMEM_SIZE_SCALE=<num>
123 This configures the amount of kernel virtual address (KVA) space to
124 dedicate to the kmem_arena map.
125 The scale value is the ratio of physical to virtual pages.
126 The default value of 3 allocates a page of KVA for each 3 pages
127 of physical ram in the system.
129 The kernel and modules, including the root image, also consume KVA.
130 The combination of a large root image and the default scaling
131 may preallocate so much KVA that there is not enough
132 remaining address space to allocate kernel stacks, IO buffers,
133 and other resources that are not part of kmem_arena.
134 Overallocating kmem_arena space is likely to manifest as failure to
135 launch userland processes with "cannot allocate kernel stack" messages.
137 Setting the scale value too high may result in kernel failure to allocate
138 memory because kmem_arena is too small, and the failure may require
139 significant runtime to manifest.
140 Empirically, a value of 5 works well for a 200 MiB root image on
141 a system with 2 GiB of physical ram.
153 driver first appeared in
155 as a cleaner replacement
156 for the MFS functionality previously used in
160 installation process.
164 driver did a hostile takeover of the
171 driver was written by
172 .An Poul-Henning Kamp Aq Mt phk@FreeBSD.org .