virtio: flush buffers on open

Fix bug found by Christian Borntraeger: if the other side fills all
the registered network buffers before we enable NAPI, we will never
get an interrupt.  The simplest fix is to process the input queue once
on open.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/net/virtio_net.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff -r 1dd61213039c drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c	Tue Dec 18 17:34:18 2007 +1100
+++ b/drivers/net/virtio_net.c	Tue Dec 18 17:47:39 2007 +1100
@@ -326,6 +326,13 @@ static int virtnet_open(struct net_devic
 		return -ENOMEM;
 
 	napi_enable(&vi->napi);
+
+	/* If all buffers were filled by other side before we napi_enabled, we
+	 * won't get another interrupt, so process any outstanding packets
+	 * now.  virtnet_poll wants re-enable the queue, so we disable here. */
+	vi->rvq->vq_ops->disable_cb(vi->rvq);
+	netif_rx_schedule(vi->dev, &vi->napi);
+
 	return 0;
 }