From 92045d80badc43c9f95897aad675dc7ef17a3b3f Mon Sep 17 00:00:00 2001
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Mon, 23 Apr 2012 14:11:14 +0300
Subject: [PATCH] virtio: add missing mb() on enable notification
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 <mst@redhat.com>
---
hw/virtio.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/hw/virtio.c b/hw/virtio.c
index bb994c9b51..5615b59a6c 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)
--
2.30.2