virtio-net: fix bottom-half packet TX on asynchronous completion

When virtio-net is used with the socket netdev backend, the backend
can be busy and not able to collect new packets.

In this case, net_socket_receive() returns 0 and registers a poll function
to detect when the socket is ready again.

In virtio_net_tx_bh(), virtio_net_flush_tx() forwards the 0, the virtio
notifications are disabled and the function is not re-scheduled, waiting
for the backend to be ready.

When the socket netdev backend is again able to send packets, the poll
function re-starts to flush remaining packets. This is done by
calling virtio_net_tx_complete(). It re-enables notifications and calls
again virtio_net_flush_tx().

But it seems if virtio_net_flush_tx() reaches the tx_burst value all
the queue is not flushed and no new notification is sent to re-schedule
virtio_net_tx_bh(). Nothing re-start to flush the queue and remaining
packets are stuck in the queue.

To fix that, detect in virtio_net_tx_complete() if virtio_net_flush_tx()
has been stopped by tx_burst and if yes re-schedule the bottom half
function virtio_net_tx_bh() to flush the remaining packets.

This is what is done in virtio_net_tx_bh() when the virtio_net_flush_tx()
is synchronous, and completly by-passed when the operation needs to be
asynchronous.

Fixes: a697a334b3c4 ("virtio-net: Introduce a new bottom half packet TX")
Cc: alex.williamson@redhat.com
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/virtio-net.c

@@ -2526,6 +2526,7 @@ static void virtio_net_tx_complete(NetClientState *nc, ssize_t len)
     VirtIONet *n = qemu_get_nic_opaque(nc);
     VirtIONetQueue *q = virtio_net_get_subqueue(nc);
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
+    int ret;
 
     virtqueue_push(q->tx_vq, q->async_tx.elem, 0);
     virtio_notify(vdev, q->tx_vq);
@@ -2534,7 +2535,17 @@ static void virtio_net_tx_complete(NetClientState *nc, ssize_t len)
     q->async_tx.elem = NULL;
 
     virtio_queue_set_notification(q->tx_vq, 1);
-    virtio_net_flush_tx(q);
+    ret = virtio_net_flush_tx(q);
+    if (q->tx_bh && ret >= n->tx_burst) {
+        /*
+         * the flush has been stopped by tx_burst
+         * we will not receive notification for the
+         * remainining part, so re-schedule
+         */
+        virtio_queue_set_notification(q->tx_vq, 0);
+        qemu_bh_schedule(q->tx_bh);
+        q->tx_waiting = 1;
+    }
 }
 
 /* TX */