veth: add netpoll support

It is trivial to add netpoll support to veth, since
it is not a stacked device, we don't need to setup and
clean up netpoll.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -248,6 +248,21 @@ static void veth_dev_free(struct net_device *dev)
 	free_netdev(dev);
 }
 
+#ifdef CONFIG_NET_POLL_CONTROLLER
+static void veth_poll_controller(struct net_device *dev)
+{
+	/* veth only receives frames when its peer sends one
+	 * Since it's a synchronous operation, we are guaranteed
+	 * never to have pending data when we poll for it so
+	 * there is nothing to do here.
+	 *
+	 * We need this though so netpoll recognizes us as an interface that
+	 * supports polling, which enables bridge devices in virt setups to
+	 * still use netconsole
+	 */
+}
+#endif	/* CONFIG_NET_POLL_CONTROLLER */
+
 static const struct net_device_ops veth_netdev_ops = {
 	.ndo_init            = veth_dev_init,
 	.ndo_open            = veth_open,
@@ -257,6 +272,9 @@ static const struct net_device_ops veth_netdev_ops = {
 	.ndo_get_stats64     = veth_get_stats64,
 	.ndo_set_rx_mode     = veth_set_multicast_list,
 	.ndo_set_mac_address = eth_mac_addr,
+#ifdef CONFIG_NET_POLL_CONTROLLER
+	.ndo_poll_controller	= veth_poll_controller,
+#endif
 };
 
 #define VETH_FEATURES (NETIF_F_SG | NETIF_F_FRAGLIST | NETIF_F_ALL_TSO |    \