Subject: Re: [net PATCH 1/2] virtio_net: cap mtu when XDP programs are running

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 5 Jan 2017 05:18:50 +0200

On Wed, Jan 04, 2017 at 07:11:18PM -0800, John Fastabend wrote:
> XDP programs can not consume multiple pages so we cap the MTU to
> avoid this case. Virtio-net however only checks the MTU at XDP
> program load and does not block MTU changes after the program
> has loaded.

Do drivers really have to tweak max mtu all the time?
Seems strange, I would say drivers just report device caps
and net core enforces rules.
Can't net core do these checks?

> 
> This patch sets/clears the max_mtu value at XDP load/unload time.
> 
> Signed-off-by: John Fastabend <[email-masked]>
> ---
>  drivers/net/virtio_net.c |   26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4a10500..261103d9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1699,11 +1699,28 @@ static void virtnet_init_settings(struct net_device *dev)
>  	.set_settings = virtnet_set_settings,
>  };
>  
> +#define MIN_MTU ETH_MIN_MTU
> +#define MAX_MTU ETH_MAX_MTU
> +
> +static unsigned long int virtnet_xdp_mtu(struct bpf_prog *prog,
> +					 struct virtnet_info *vi)
> +{
> +	if (!prog && virtio_has_feature(vi->vdev, VIRTIO_NET_F_MTU))
> +		return virtio_cread16(vi->vdev,
> +				      offsetof(struct virtio_net_config, mtu));
> +	else if (!prog)
> +		return ETH_MAX_MTU;
> +	else if (vi->mergeable_rx_bufs)
> +		return PAGE_SIZE - sizeof(struct padded_vnet_hdr);
> +	else
> +		return GOOD_PACKET_LEN;
> +}
> +
>  static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>  {
> -	unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);
>  	struct virtnet_info *vi = netdev_priv(dev);
>  	struct bpf_prog *old_prog;
> +	unsigned long int max_sz;
>  	u16 xdp_qp = 0, curr_qp;
>  	int i, err;
>  
> @@ -1720,6 +1737,7 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>  		return -EINVAL;
>  	}
>  
> +	max_sz = virtnet_xdp_mtu(prog, vi);
>  	if (dev->mtu > max_sz) {
>  		netdev_warn(dev, "XDP requires MTU less than %lu\n", max_sz);
>  		return -EINVAL;
> @@ -1748,6 +1766,9 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>  			virtnet_set_queues(vi, curr_qp);
>  			return PTR_ERR(prog);
>  		}
> +		dev->max_mtu = max_sz;
> +	} else {
> +		dev->max_mtu = ETH_MAX_MTU;
>  	}
>  
>  	vi->xdp_queue_pairs = xdp_qp;
> @@ -2133,9 +2154,6 @@ static bool virtnet_validate_features(struct virtio_device *vdev)
>  	return true;
>  }
>  
> -#define MIN_MTU ETH_MIN_MTU
> -#define MAX_MTU ETH_MAX_MTU
> -
>  static int virtnet_probe(struct virtio_device *vdev)
>  {
>  	int i, err;