[Qemu-devel] [PATCHv4 2/3] virtio: add missing mb() on enable notification

[Michael S. Tsirkin]

This fixes an issue dual to the one fixed by
patch 'virtio: add missing mb() on notification'
and applies on top.

In this case, to enable vq kick to exit to host,
qemu writes out used flag then reads the
avail index. if these are reordered we get a race:

    host avail index read: ring is empty
                guest avail index write
                guest flag read: exit disabled
    host used flag write: enable exit

which results in a lost exit: host will never be notified about the
avail index update.  Again, happens in the field but only seems to
trigger on some specific hardware.

Insert an smp_mb barrier operation to ensure the correct ordering.

Signed-off-by: Michael S. Tsirkin <address@hidden>
---
 hw/virtio.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/hw/virtio.c b/hw/virtio.c
index 8defd80..def0bf1 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -209,6 +209,10 @@ void virtio_queue_set_notification(VirtQueue *vq, int 
enable)
     } else {
         vring_used_flags_set_bit(vq, VRING_USED_F_NO_NOTIFY);
     }
+    if (enable) {
+        /* Expose avail event/used flags before caller checks the avail idx. */
+        smp_mb();
+    }
 }
 
 int virtio_queue_ready(VirtQueue *vq)
-- 
MST