netdev - Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP

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Date:   Wed, 6 May 2020 03:53:31 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet
 header for XDP

On Wed, May 06, 2020 at 02:16:32PM +0800, Jason Wang wrote:
> We tried to reserve space for vnet header before
> xdp.data_hard_start. But this is useless since the packet could be
> modified by XDP which may invalidate the information stored in the
> header and there's no way for XDP to know the existence of the vnet
> header currently.

What do you mean? Doesn't XDP_PASS use the header in the buffer?

> So let's just not reserve space for vnet header in this case.

In any case, we can find out XDP does head adjustments
if we need to.


> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/net/virtio_net.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 11f722460513..98dd75b665a5 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -684,8 +684,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  			page = xdp_page;
>  		}
>  
> -		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
> -		xdp.data = xdp.data_hard_start + xdp_headroom;
> +		xdp.data_hard_start = buf + VIRTNET_RX_PAD;
> +		xdp.data = xdp.data_hard_start + xdp_headroom + vi->hdr_len;
>  		xdp.data_end = xdp.data + len;
>  		xdp.data_meta = xdp.data;
>  		xdp.rxq = &rq->xdp_rxq;
> @@ -845,7 +845,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  		 * the descriptor on if we get an XDP_TX return code.
>  		 */
>  		data = page_address(xdp_page) + offset;
> -		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
> +		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM;
>  		xdp.data = data + vi->hdr_len;
>  		xdp.data_end = xdp.data + (len - vi->hdr_len);
>  		xdp.data_meta = xdp.data;
> -- 
> 2.20.1