[PATCH v3] virtio-net: Add initial MTU advice feature

[Aaron Conole]

This commit adds the feature bit and associated mtu device entry for the
virtio network device.  When a virtio device comes up, it checks the
feature bit for the VIRTIO_NET_F_MTU feature.  If such feature bit is
enabled, the driver will read the advised MTU and use it as the initial
value.

Signed-off-by: Aaron Conole <acon...@redhat.com>
---
v2->v3:
* Added a check for the MTU, and a bit clear for it being out of range.

 drivers/net/virtio_net.c        | 10 ++++++++++
 include/uapi/linux/virtio_net.h |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e0638e5..192f321 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1780,6 +1780,7 @@ static int virtnet_probe(struct virtio_device *vdev)
        struct net_device *dev;
        struct virtnet_info *vi;
        u16 max_queue_pairs;
+       int mtu;
 
        if (!vdev->config->get) {
                dev_err(&vdev->dev, "%s failure: config access disabled\n",
@@ -1896,6 +1897,14 @@ 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)) {
+               mtu = virtio_cread16(vdev,
+                                    offsetof(struct virtio_net_config,
+                                             mtu));
+               if (virtnet_change_mtu(dev, mtu))
+                       __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
+       }
+
        if (vi->any_header_sg)
                dev->needed_headroom = vi->hdr_len;
 
@@ -2067,6 +2076,7 @@ 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 = {
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index ec32293..1ab4ea6 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    /* Initial MTU advice */
 
 #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