Subject: Re: [RFC v2 -next 2/2] virtio_net: Read the advised MTU

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 16 Mar 2016 16:24:46 +0200

On Tue, Mar 15, 2016 at 05:04:13PM -0400, Aaron Conole wrote:
> 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.

I don't see this as an error. Device might at best give a hint,
user/network admin always knows best.

> 
> Signed-off-by: Aaron Conole <[email-masked]>
> ---
> v2:
> * Whitespace cleanup in the last hunk
> * Code style change around the pr_warn
> * Additional test for mtu change before printing warning
> 
>  drivers/net/virtio_net.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 767ab11..429fe01 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -146,6 +146,7 @@ struct virtnet_info {
>  	virtio_net_ctrl_ack ctrl_status;
>  	u8 ctrl_promisc;
>  	u8 ctrl_allmulti;
> +	bool negotiated_mtu;
>  };
>  
>  struct padded_vnet_hdr {
> @@ -1390,8 +1391,11 @@ 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) && (dev->mtu != new_mtu))
> +		pr_warn("changing mtu while the advised mtu bit exists.");

I don't really see why are we warning here. Just drop this chunk,
as well as the flag in struct virtnet_info.

>  	dev->mtu = new_mtu;
>  	return 0;
>  }
> @@ -1836,6 +1840,13 @@ 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)) {
> +		vi->negotiated_mtu = true;
> +		dev->mtu = virtio_cread16(vdev,
> +					  offsetof(struct virtio_net_config,
> +						   mtu));
> +	}
> +
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> @@ -2019,6 +2030,7 @@ 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 = {
> -- 
> 2.5.0