virtio-net: purge outstanding packets when starting vhost

whenever we start vhost, virtio could have outstanding packets
queued, when they complete later we'll modify the ring
while vhost is processing it.

To prevent this, purge outstanding packets on vhost start.

Cc: qemu-stable@nongnu.org
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 086abc1)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/net/virtio-net.c

@@ -125,10 +125,23 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
         return;
     }
     if (!n->vhost_started) {
-        int r;
+        int r, i;
+
         if (!vhost_net_query(get_vhost_net(nc->peer), vdev)) {
             return;
         }
+
+        /* Any packets outstanding? Purge them to avoid touching rings
+         * when vhost is running.
+         */
+        for (i = 0;  i < queues; i++) {
+            NetClientState *qnc = qemu_get_subqueue(n->nic, i);
+
+            /* Purge both directions: TX and RX. */
+            qemu_net_queue_purge(qnc->peer->incoming_queue, qnc);
+            qemu_net_queue_purge(qnc->incoming_queue, qnc->peer);
+        }
+
         n->vhost_started = 1;
         r = vhost_net_start(vdev, n->nic->ncs, queues);
         if (r < 0) {