This patch checks the feature bit for the VIRTIO_NET_F_MTU feature. If it
exists, read the advised MTU and use it.

No proper error handling is provided for the case where a user changes the
negotiated MTU. A future commit will add proper error handling. Instead, a
warning is emitted if the guest changes the device MTU after previously being
given advice.

Signed-off-by: Aaron Conole <aconole@redhat.com>
---
  drivers/net/virtio_net.c | 15 ++++++++++++++-
  1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 767ab11..7175563 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1390,8 +1391,12 @@ static const struct ethtool_ops virtnet_ethtool_ops = {

  static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
  {
+	struct virtnet_info *vi = netdev_priv(dev);
  	if (new_mtu < MIN_MTU || new_mtu > MAX_MTU)
  		return -EINVAL;
+	if (vi->negotiated_mtu == true) {
+		pr_warn("changing mtu from negotiated mtu.");
+	}