virtio_net: flush uncompleted TX on reset

virtio_net: flush uncompleted TX on reset

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 with an hubport backend
that is not connected to a functional network, eg,

 -device virtio-net-pci,netdev=hub0 -netdev hubport,id=hub0,hubid=0

and no other -netdev hubport,hubid=0 on the command line.

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.

CC: qemu-stable@nongnu.org
Reported-by: R. Nageswara Sastry <nasastry@in.ibm.com>
Buglink: https://github.com/open-power-host-os/qemu/issues/37
Signed-off-by: Greg Kurz <groug@kaod.org>
Tested-by: R. Nageswara Sastry <nasastry@in.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>