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

From: Aaron Conole <[email-masked]>
Date: Tue, 15 Mar 2016 16:52:39 -0400

Paolo Abeni <[email-masked]> writes:

> 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 <[email-masked]>
>> ---
>>  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))
>
> ?

Okay, I'll put this test in.

>> +		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

D'oh! Okay, v2 will have this fixed.

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

Thanks so much for the review, Paolo!

-Aaron