vhost: restore avail index from vring used index on disconnection

vhost_virtqueue_stop() gets avail index value from the backend,
except if the backend is not responding.

It happens when the backend crashes, and in this case, internal
state of the virtio queue is inconsistent, making packets
to corrupt the vring state.

With a Linux guest, it results in following error message on
backend reconnection:

[   22.444905] virtio_net virtio0: output.0:id 0 is not a head!
[   22.446746] net enp0s3: Unexpected TXQ (0) queue failure: -5
[   22.476360] net enp0s3: Unexpected TXQ (0) queue failure: -5

Fixes: 283e2c2adcb8 ("net: virtio-net discards TX data after link down")
Cc: qemu-stable@nongnu.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 2ae39a113af311cb56a0c35b7f212dafcef15303)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index b737ca915b0698e722876772219afb4488917d2d..76f6e1fcaa44b7e7b993ec97962f605ebf4af99f 100644 (file)
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1137,6 +1137,10 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
     r = dev->vhost_ops->vhost_get_vring_base(dev, &state);
     if (r < 0) {
         VHOST_OPS_DEBUG("vhost VQ %d ring restore failed: %d", idx, r);
+        /* Connection to the backend is broken, so let's sync internal
+         * last avail idx to the device used idx.
+         */
+        virtio_queue_restore_last_avail_idx(vdev, idx);
     } else {
         virtio_queue_set_last_avail_idx(vdev, idx, state.num);
     }