virtio_net: large tx MTU support

We don't really have a max tx packet size limit, so allow configuring
the device with up to 64k tx MTU.
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/net/virtio_net.c

@@ -613,6 +613,17 @@ static struct ethtool_ops virtnet_ethtool_ops = {
 	.set_tso = ethtool_op_set_tso,
 };
 
+#define MIN_MTU 68
+#define MAX_MTU 65535
+
+static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
+{
+	if (new_mtu < MIN_MTU || new_mtu > MAX_MTU)
+		return -EINVAL;
+	dev->mtu = new_mtu;
+	return 0;
+}
+
 static int virtnet_probe(struct virtio_device *vdev)
 {
 	int err;
@@ -628,6 +639,7 @@ static int virtnet_probe(struct virtio_device *vdev)
 	dev->open = virtnet_open;
 	dev->stop = virtnet_close;
 	dev->hard_start_xmit = start_xmit;
+	dev->change_mtu = virtnet_change_mtu;
 	dev->features = NETIF_F_HIGHDMA;
 #ifdef CONFIG_NET_POLL_CONTROLLER
 	dev->poll_controller = virtnet_netpoll;