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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>
This commit is contained in:
Michael S. Tsirkin 2012-04-23 15:46:22 +03:00
parent 92045d80ba
commit a821ce5933
2 changed files with 17 additions and 2 deletions
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5
hw/virtio.c
View File

@ -287,6 +287,11 @@ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
idx, vring_avail_idx(vq));
exit(1);
}
/* On success, callers read a descriptor at vq->last_avail_idx.
* Make sure descriptor read does not bypass avail index read. */
if (num_heads) {
smp_rmb();
}
return num_heads;
}

14
qemu-barrier.h
View File

@ -7,12 +7,13 @@
#if defined(__i386__)
/*
* Because of the strongly ordered x86 storage model, wmb() is a nop
* 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.
@ -27,6 +28,7 @@
#elif defined(__x86_64__)
#define smp_wmb() barrier()
#define smp_rmb() barrier()
#define smp_mb() asm volatile("mfence" ::: "memory")
#elif defined(_ARCH_PPC)
@ -37,6 +39,13 @@
* 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
@ -45,10 +54,11 @@
* 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().
* be overkill for wmb() and rmb().
*/
#define smp_wmb() __sync_synchronize()
#define smp_mb() __sync_synchronize()
#define smp_rmb() __sync_synchronize()
#endif