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

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Date:	Thu, 17 Mar 2016 17:20:05 -0400
From:	Aaron Conole <aconole@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 -next 2/2] virtio_net: Read the advised MTU

"Michael S. Tsirkin" <mst@...hat.com> writes:

> 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 <aconole@...heb.org>
>> ---
>> 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.

Okay. I was warning because the user is changing this after telling to
use something different - but if you don't think it's an error, I will
drop it.

Thanks so much for the review, Michael!

-Aaron

>>  	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