Sibyte provides a 64-bit read-only counter that counts at half the processor

Sibyte provides a 64-bit read-only counter that counts at half the processor
frequency. This counter can be accessed coherently from both cores.

Use this as the preferred timecounter for the SWARM kernels.

The CP0 COUNT register is unusable as the timecounter on SMP platforms because
the COUNT registers on different CPUs are not guaranteed to be in sync.