virtio_net: Fix stalled inbound traffic on early packets

The current virtio_net driver has a startup race, which prevents any incoming traffic:

If try_fill_recv submits buffers to the host system data might be filled in and an interrupt is sent, before napi_enable finishes. In that case the interrupt will kick skb_recv_done which will then call netif_rx_schedule. netif_rx_schedule checks, if NAPI_STATE_SCHED is set - which is not as we did not run napi_enable. No poll routine is scheduled. Furthermore, skb_recv_done returns false, we disables interrupts for this device.

One solution is the enable napi before inbound buffer are available.

---
 drivers/net/virtio_net.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: kvm/drivers/net/virtio_net.c
===================================================================
@@ -285,13 +285,15 @@ static int virtnet_open(struct net_devic
 {
 	struct virtnet_info *vi = netdev_priv(dev);
 
+	napi_enable(&vi->napi);
 	try_fill_recv(vi);
 
 	/* If we didn't even get one input buffer, we're useless. */
-	if (vi->num == 0)
+	if (vi->num == 0) {
+		napi_disable(&vi->napi);
 		return -ENOMEM;
+	}
 
-	napi_enable(&vi->napi);
 	return 0;
 }