net: notify iothread after flushing queue

virtio-net has code to flush the queue and notify the iothread
whenever new receive buffers are added by the guest.  That is
fine, and indeed we need to do the same in all other drivers.
However, notifying the iothread should be work for the network
subsystem.  And since we are at it we can add a little smartness:
if some of the queued packets already could not be delivered,
there is no need to notify the iothread.

Reported-by: Luigi Rizzo <rizzo@iet.unipi.it>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/virtio-net.c

@@ -447,10 +447,6 @@ static void virtio_net_handle_rx(VirtIODevice *vdev, VirtQueue *vq)
     VirtIONet *n = to_virtio_net(vdev);
 
     qemu_flush_queued_packets(&n->nic->nc);
-
-    /* We now have RX buffers, signal to the IO thread to break out of the
-     * select to re-poll the tap file descriptor */
-    qemu_notify_event();
 }
 
 static int virtio_net_can_receive(NetClientState *nc)

net.c

@@ -357,7 +357,12 @@ void qemu_flush_queued_packets(NetClientState *nc)
 {
     nc->receive_disabled = 0;
 
-    qemu_net_queue_flush(nc->send_queue);
+    if (qemu_net_queue_flush(nc->send_queue)) {
+        /* We emptied the queue successfully, signal to the IO thread to repoll
+         * the file descriptor (for tap, for example).
+         */
+        qemu_notify_event();
+    }
 }
 
 static ssize_t qemu_send_packet_async_with_flags(NetClientState *sender,

net/queue.c

@@ -228,7 +228,7 @@ void qemu_net_queue_purge(NetQueue *queue, NetClientState *from)
     }
 }
 
-void qemu_net_queue_flush(NetQueue *queue)
+bool qemu_net_queue_flush(NetQueue *queue)
 {
     while (!QTAILQ_EMPTY(&queue->packets)) {
         NetPacket *packet;
@@ -244,7 +244,7 @@ void qemu_net_queue_flush(NetQueue *queue)
                                      packet->size);
         if (ret == 0) {
             QTAILQ_INSERT_HEAD(&queue->packets, packet, entry);
-            break;
+            return false;
         }
 
         if (packet->sent_cb) {
@@ -253,4 +253,5 @@ void qemu_net_queue_flush(NetQueue *queue)
 
         g_free(packet);
     }
+    return true;
 }

net/queue.h

@@ -53,6 +53,6 @@ ssize_t qemu_net_queue_send_iov(NetQueue *queue,
                                 NetPacketSent *sent_cb);
 
 void qemu_net_queue_purge(NetQueue *queue, NetClientState *from);
-void qemu_net_queue_flush(NetQueue *queue);
+bool qemu_net_queue_flush(NetQueue *queue);
 
 #endif /* QEMU_NET_QUEUE_H */