[PATCH] virtio: remove unnecessary thread fence while reading next descriptor

[Ilya Maximets]

It was supposed to be a compiler barrier and it was a compiler barrier
initially called 'wmb' (??) when virtio core support was introduced.
Later all the instances of 'wmb' were switched to smp_wmb to fix memory
ordering issues on non-x86 platforms.  However, this one doesn't need
to be an actual barrier.  It's enough for it to stay a compiler barrier
as its only purpose is to ensure that the value is not read twice.

There is no counterpart read barrier in the drivers, AFAICT.  And even
if we needed an actual barrier, it shouldn't have been a write barrier.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
---
 hw/virtio/virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 309038fd46..6eb8586858 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1051,7 +1051,7 @@ static int virtqueue_split_read_next_desc(VirtIODevice 
*vdev, VRingDesc *desc,
     /* Check they're not leading us off end of descriptors. */
     *next = desc->next;
     /* Make sure compiler knows to grab that: we don't want it changing! */
-    smp_wmb();
+    barrier();
 
     if (*next >= max) {
         virtio_error(vdev, "Desc next is %u", *next);
-- 
2.40.1