Subject: Re: [net-next PATCH v5 1/6] net: virtio dynamically disable/enable LRO

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Dec 2016 23:36:34 +0200

On Wed, Dec 07, 2016 at 12:11:11PM -0800, John Fastabend wrote:
> This adds support for dynamically setting the LRO feature flag. The
> message to control guest features in the backend uses the
> CTRL_GUEST_OFFLOADS msg type.
> 
> Signed-off-by: John Fastabend <[email-masked]>
> ---
>  drivers/net/virtio_net.c |   40 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a21d93a..a5c47b1 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1419,6 +1419,36 @@ static void virtnet_init_settings(struct net_device *dev)
>  	.set_settings = virtnet_set_settings,
>  };
>  
> +static int virtnet_set_features(struct net_device *netdev,
> +				netdev_features_t features)
> +{
> +	struct virtnet_info *vi = netdev_priv(netdev);
> +	struct virtio_device *vdev = vi->vdev;
> +	struct scatterlist sg;
> +	u64 offloads = 0;
> +
> +	if (features & NETIF_F_LRO)
> +		offloads |= (1 << VIRTIO_NET_F_GUEST_TSO4) |
> +			    (1 << VIRTIO_NET_F_GUEST_TSO6);
> +
> +	if (features & NETIF_F_RXCSUM)
> +		offloads |= (1 << VIRTIO_NET_F_GUEST_CSUM);
> +
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS)) {
> +		sg_init_one(&sg, &offloads, sizeof(uint64_t));
> +		if (!virtnet_send_command(vi,
> +					  VIRTIO_NET_CTRL_GUEST_OFFLOADS,
> +					  VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET,
> +					  &sg)) {

Hmm I just realised that this will slow down setups that bridge
virtio net interfaces since bridge calls this if provided.
See below.

> +			dev_warn(&netdev->dev,
> +				 "Failed to set guest offloads by virtnet command.\n");
> +			return -EINVAL;
> +		}
> +	}

Hmm if VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is off, this fails
silently. It might actually be a good idea to avoid
breaking setups.

> +
> +	return 0;
> +}
> +
>  static const struct net_device_ops virtnet_netdev = {
>  	.ndo_open            = virtnet_open,
>  	.ndo_stop   	     = virtnet_close,
> @@ -1435,6 +1465,7 @@ static void virtnet_init_settings(struct net_device *dev)
>  #ifdef CONFIG_NET_RX_BUSY_POLL
>  	.ndo_busy_poll		= virtnet_busy_poll,
>  #endif
> +	.ndo_set_features	= virtnet_set_features,
>  };
>  
>  static void virtnet_config_changed_work(struct work_struct *work)
> @@ -1815,6 +1846,12 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM))
>  		dev->features |= NETIF_F_RXCSUM;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) &&
> +	    virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6)) {
> +		dev->features |= NETIF_F_LRO;
> +		dev->hw_features |= NETIF_F_LRO;

So the issue is I think that the virtio "LRO" isn't really
LRO, it's typically just GRO forwarded to guests.
So these are easily re-split along MTU boundaries,
which makes it ok to forward these across bridges.

It's not nice that we don't document this in the spec,
but it's the reality and people rely on this.

For now, how about doing a custom thing and just disable/enable
it as XDP is attached/detached?

> +	}
> +
>  	dev->vlan_features = dev->features;
>  
>  	/* MTU range: 68 - 65535 */
> @@ -2057,7 +2094,8 @@ static int virtnet_restore(struct virtio_device *vdev)
>  	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_MTU
> +	VIRTIO_NET_F_MTU, \
> +	VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,