[Qemu-devel] [PATCH] qemu/virtio: make wmb compiler barrier + comments

[Michael S. Tsirkin]

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 <address@hidden>
---
 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