mirror of
https://github.com/xemu-project/xemu.git
synced 2024-11-23 19:49:43 +00:00
a821ce5933
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>
66 lines
1.8 KiB
C
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
|