linux-kernel - Re: [RFC -next 2/2] virtio_net: Read and use the advised MTU

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Date:	Thu, 10 Mar 2016 15:57:28 +0100
From:	Paolo Abeni <pabeni@...hat.com>
To:	Aaron Conole <aconole@...hat.com>
Cc:	netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC -next 2/2] virtio_net: Read and use the advised MTU

On Thu, 2016-03-10 at 09:28 -0500, 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.
> 
> Signed-off-by: Aaron Conole <aconole@...hat.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
> @@ -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 +1391,12 @@ 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) {

why don't:

if ((vi->negotiated_mtu == true) && (dev->mtu != new_mtu))

?

> +		pr_warn("changing mtu from negotiated mtu.");
> +	}
>  	dev->mtu = new_mtu;
>  	return 0;
>  }
> @@ -1836,6 +1841,13 @@ 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;
>  
> @@ -2017,8 +2029,9 @@ static unsigned int features[] = {
>  	VIRTIO_NET_F_MRG_RXBUF, VIRTIO_NET_F_STATUS, VIRTIO_NET_F_CTRL_VQ,
>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ,
> -	VIRTIO_NET_F_CTRL_MAC_ADDR,
> +	VIRTIO_NET_F_CTRL_MAC_ADDR, 

Here a trailing white space slipped-in.

Otherwise LGTM.

Paolo

>  	VIRTIO_F_ANY_LAYOUT,
> +	VIRTIO_NET_F_MTU,
>  };
>  
>  static struct virtio_driver virtio_net_driver = {