wmb must be at least a compiler barrier, even without SMP. Further, we likely need some rmb()/mb() as well: I have not audited the code but lguest has mb(), add a comment for now.
Signed-off-by: Michael S. Tsirkin <m...@redhat.com> --- hw/virtio.c | 5 ++++- 1 files changed, 4 insertions(+), 1 deletions(-) diff --git a/hw/virtio.c b/hw/virtio.c index 337ff27..1f92171 100644 --- a/hw/virtio.c +++ b/hw/virtio.c @@ -23,8 +23,11 @@ /* QEMU doesn't strictly need write barriers since everything runs in * lock-step. We'll leave the calls to wmb() in though to make it obvious for * KVM or if kqemu gets SMP support. + * In any case, we must prevent the compiler from reordering the code. + * TODO: we likely need some rmb()/mb() as well. */ -#define wmb() do { } while (0) + +#define wmb() __asm__ __volatile__("": : :"memory") typedef struct VRingDesc { -- 1.6.5.rc2