linux-kernel - Re: [PATCH net v2 2/2] virtio-net: xsk: rx: move the xdp->data adjustment to buf_to

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Message-ID: <e6654755-3aa1-4f4b-a6ab-c7568d8a5d4e@redhat.com>
Date: Tue, 24 Jun 2025 12:17:53 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.com>, netdev@...r.kernel.org
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net v2 2/2] virtio-net: xsk: rx: move the xdp->data
 adjustment to buf_to_xdp()

On 6/21/25 4:49 PM, Bui Quang Minh wrote:
> This commit does not do any functional changes. It moves xdp->data
> adjustment for buffer other than first buffer to buf_to_xdp() helper so
> that the xdp_buff adjustment does not scatter over different functions.
> 
> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
> ---
>  drivers/net/virtio_net.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 1eb237cd5d0b..4e942ea1bfa3 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1159,7 +1159,19 @@ static struct xdp_buff *buf_to_xdp(struct virtnet_info *vi,
>  		return NULL;
>  	}
>  
> -	xsk_buff_set_size(xdp, len);
> +	if (first_buf) {
> +		xsk_buff_set_size(xdp, len);
> +	} else {
> +		/* This is the same as xsk_buff_set_size but with the adjusted
> +		 * xdp->data.
> +		 */
> +		xdp->data = xdp->data_hard_start + XDP_PACKET_HEADROOM;
> +		xdp->data -= vi->hdr_len;
> +		xdp->data_meta = xdp->data;
> +		xdp->data_end = xdp->data + len;
> +		xdp->flags = 0;
> +	}
> +
>  	xsk_buff_dma_sync_for_cpu(xdp);
>  
>  	return xdp;
> @@ -1284,7 +1296,7 @@ static int xsk_append_merge_buffer(struct virtnet_info *vi,
>  			goto err;
>  		}
>  
> -		memcpy(buf, xdp->data - vi->hdr_len, len);
> +		memcpy(buf, xdp->data, len);
>  
>  		xsk_buff_free(xdp);
>  

I'm unsure if this change is in the right direction because it almost
open-code the existing xsk_buff_set_size() helper - any changes there
should be reflected here, too.

Also AFAICS xdp->data will now carry a different value, and I guess such
change is user-visible from the attached xdp program. The commit message
should at least mentions such fact.

Thanks,

Paolo