From: Michael S. Tsirkin <mst@redhat.com>
Date: Mon, 26 Oct 2009 13:17:15 +0000 (+0200)
Subject: qemu/virtio: make wmb compiler barrier + comments
X-Git-Tag: TizenStudio_2.0_p2.3~6804
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=a316810d1c64dbb1d687a75f872d442afa73c29a;p=sdk%2Femulator%2Fqemu.git

qemu/virtio: make wmb compiler barrier + comments

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 <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---

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
 {