[RFC v3 -next 2/2] virtio_net: Read the advised MTU

[Aaron Conole <aconole@redhat.com>]

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@bytheb.org>
---
v2:
* Whitespace cleanup in the last hunk
* Code style change around the pr_warn
* Additional test for mtu change before printing warning
v3:
* removed the mtu change warning

 drivers/net/virtio_net.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 49d84e5..2308083 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1450,6 +1450,7 @@ 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;
 	dev->mtu = new_mtu;
@@ -1896,6 +1897,12 @@ static int virtnet_probe(struct virtio_device *vdev)
 	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
 		vi->has_cvq = true;
 
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
+		dev->mtu = virtio_cread16(vdev,
+					  offsetof(struct virtio_net_config,
+						   mtu));
+	}
+
 	if (vi->any_header_sg)
 		dev->needed_headroom = vi->hdr_len;
 
@@ -2081,6 +2088,7 @@ static unsigned int features[] = {
 	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ,
 	VIRTIO_NET_F_CTRL_MAC_ADDR,
 	VIRTIO_F_ANY_LAYOUT,
+	VIRTIO_NET_F_MTU,
 };
 
 static struct virtio_driver virtio_net_driver = {
-- 
2.5.5