qemu/qemu-barrier.h
Michael S. Tsirkin a821ce5933 virtio: order index/descriptor reads
virtio has the equivalent of:

	if (vq->last_avail_index != vring_avail_idx(vq)) {
		read descriptor head at vq->last_avail_index;
	}

In theory, processor can reorder descriptor head
read to happen speculatively before the index read.
this would trigger the following race:

	host descriptor head read <- reads invalid head from ring
		guest writes valid descriptor head
		guest writes avail index
	host avail index read <- observes valid index

as a result host will use an invalid head value.
This was not observed in the field by me but after
the experience with the previous two races
I think it is prudent to address this theoretical race condition.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2012-04-25 10:53:47 +03:00

66 lines
1.8 KiB
C

#ifndef __QEMU_BARRIER_H
#define __QEMU_BARRIER_H 1
/* Compiler barrier */
#define barrier() asm volatile("" ::: "memory")
#if defined(__i386__)
/*
* Because of the strongly ordered x86 storage model, wmb() and rmb() are nops
* on x86(well, a compiler barrier only). Well, at least as long as
* qemu doesn't do accesses to write-combining memory or non-temporal
* load/stores from C code.
*/
#define smp_wmb() barrier()
#define smp_rmb() barrier()
/*
* We use GCC builtin if it's available, as that can use
* mfence on 32 bit as well, e.g. if built with -march=pentium-m.
* However, on i386, there seem to be known bugs as recently as 4.3.
* */
#if defined(__GNUC__) && __GNUC__ >= 4 && __GNUC_MINOR__ >= 4
#define smp_mb() __sync_synchronize()
#else
#define smp_mb() asm volatile("lock; addl $0,0(%%esp) " ::: "memory")
#endif
#elif defined(__x86_64__)
#define smp_wmb() barrier()
#define smp_rmb() barrier()
#define smp_mb() asm volatile("mfence" ::: "memory")
#elif defined(_ARCH_PPC)
/*
* We use an eieio() for wmb() on powerpc. This assumes we don't
* need to order cacheable and non-cacheable stores with respect to
* each other
*/
#define smp_wmb() asm volatile("eieio" ::: "memory")
#if defined(__powerpc64__)
#define smp_rmb() asm volatile("lwsync" ::: "memory")
#else
#define smp_rmb() asm volatile("sync" ::: "memory")
#endif
#define smp_mb() asm volatile("sync" ::: "memory")
#else
/*
* For (host) platforms we don't have explicit barrier definitions
* for, we use the gcc __sync_synchronize() primitive to generate a
* full barrier. This should be safe on all platforms, though it may
* be overkill for wmb() and rmb().
*/
#define smp_wmb() __sync_synchronize()
#define smp_mb() __sync_synchronize()
#define smp_rmb() __sync_synchronize()
#endif
#endif