If the backend could not transmit a packet right away for some reason,
the packet is queued for asynchronous sending. The corresponding vq
element is tracked in the async_tx.elem field of the VirtIONetQueue,
for later freeing when the transmission is complete.

If a reset happens before completion, virtio_net_tx_complete() will push
async_tx.elem back to the guest anyway, and we end up with the inuse flag
of the vq being equal to -1. The next call to virtqueue_pop() is then
likely to fail with "Virtqueue size exceeded".

This can be reproduced easily by starting a guest without a net backend,
doing a system reset when it is booted, and finally snapshotting it.

The appropriate fix is to ensure that such an asynchronous transmission
cannot survive a device reset. So for all queues, we first try to send
the packet again, and eventually we purge it if the backend still could
not deliver it.

Reported-by: R. Nageswara Sastry <nasas...@in.ibm.com>
Buglink: https://github.com/open-power-host-os/qemu/issues/37
Signed-off-by: Greg Kurz <gr...@kaod.org>
 hw/net/virtio-net.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 188744e17d57..eea3cdb2c700 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -422,6 +422,7 @@ static RxFilterInfo 
*virtio_net_query_rxfilter(NetClientState *nc)
 static void virtio_net_reset(VirtIODevice *vdev)
     VirtIONet *n = VIRTIO_NET(vdev);
+    int i;
     /* Reset back to compatibility mode */
     n->promisc = 1;
@@ -445,6 +446,16 @@ static void virtio_net_reset(VirtIODevice *vdev)
     memcpy(&n->mac[0], &n->nic->conf->macaddr, sizeof(n->mac));
     qemu_format_nic_info_str(qemu_get_queue(n->nic), n->mac);
     memset(n->vlans, 0, MAX_VLAN >> 3);
+    /* Flush any async TX */
+    for (i = 0;  i < n->max_queues; i++) {
+        NetClientState *nc = qemu_get_subqueue(n->nic, i);
+        if (!qemu_net_queue_flush(nc->peer->incoming_queue)) {
+            qemu_net_queue_purge(nc->peer->incoming_queue, nc);
+        }
+        assert(!virtio_net_get_subqueue(nc)->async_tx.elem);
+    }
 static void peer_test_vnet_hdr(VirtIONet *n)

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