[RFC v3 -next 0/2] virtio-net: Advised MTU feature

[RFC v3 -next 0/2] virtio-net: Advised MTU feature

[Aaron Conole]

The following series adds the ability for a hypervisor to set an MTU on the
guest during feature negotiation phase. This is useful for VM orchestration
when, for instance, tunneling is involved and the MTU of the various systems
should be homogenous.

The first patch adds the feature bit as described in the proposed VIRTIO spec
addition found at
https://lists.oasis-open.org/archives/virtio-dev/201603/msg00001.html
The second patch adds a user of the bit, and a warning when the guest changes
the MTU from the hypervisor advised MTU. Future patches may add more thorough
error handling.

v2:
* Whitespace and code style cleanups from Sergei Shtylyov and Paolo Abeni
* Additional test before printing a warning

v3:
* Removed the warning when changing MTU (which simplified the code)

Aaron Conole (2):
  virtio: Start feature MTU support
  virtio_net: Read the advised MTU

 drivers/net/virtio_net.c        | 8 ++++++++
 include/uapi/linux/virtio_net.h | 3 +++
 2 files changed, 11 insertions(+)

-- 
2.5.5

[RFC v3 -net 1/2] virtio: Start feature MTU support

[Aaron Conole]

This commit adds the feature bit and associated mtu device entry for the
virtio network device. Future commits will make use of these bits to support
negotiated MTU.

Signed-off-by: Aaron Conole <aconole@bytheb.org>
---
v2,v3:
* No change

 include/uapi/linux/virtio_net.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index ec32293..41a6a01 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -55,6 +55,7 @@
 #define VIRTIO_NET_F_MQ	22	/* Device supports Receive Flow
 					 * Steering */
 #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
+#define VIRTIO_NET_F_MTU 25	/* Device supports Default MTU Negotiation */
 
 #ifndef VIRTIO_NET_NO_LEGACY
 #define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
@@ -73,6 +74,8 @@ struct virtio_net_config {
 	 * Legal values are between 1 and 0x8000
 	 */
 	__u16 max_virtqueue_pairs;
+	/* Default maximum transmit unit advice */
+	__u16 mtu;
 } __attribute__((packed));
 
 /*
-- 
2.5.5

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

[Aaron Conole]

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