netdev - Re: [PATCH net-next v4 12/15] virtio_net: small: avoid double counting in xdp scenarios

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Date:   Thu, 27 Apr 2023 21:08:25 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        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-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next v4 12/15] virtio_net: small: avoid double
 counting in xdp scenarios

On Thu, Apr 27, 2023 at 11:05:31AM +0800, Xuan Zhuo wrote:
> Avoid the problem that some variables(headroom and so on) will repeat
> the calculation when process xdp.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Acked-by: Jason Wang <jasowang@...hat.com>


this is "code duplication" not "double counting".


> ---
>  drivers/net/virtio_net.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b8ec642899c9..f832ab8a3e6e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1027,11 +1027,10 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  	struct sk_buff *skb;
>  	struct bpf_prog *xdp_prog;
>  	unsigned int xdp_headroom = (unsigned long)ctx;
> -	unsigned int header_offset = VIRTNET_RX_PAD + xdp_headroom;
> -	unsigned int headroom = vi->hdr_len + header_offset;
> -	unsigned int buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> -			      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>  	struct page *page = virt_to_head_page(buf);
> +	unsigned int header_offset;
> +	unsigned int headroom;
> +	unsigned int buflen;
>  
>  	len -= vi->hdr_len;
>  	stats->bytes += len;
> @@ -1059,6 +1058,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  	rcu_read_unlock();
>  
>  skip_xdp:
> +	header_offset = VIRTNET_RX_PAD + xdp_headroom;
> +	headroom = vi->hdr_len + header_offset;
> +	buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> +		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +
>  	skb = build_skb(buf, buflen);
>  	if (!skb)
>  		goto err;
> -- 
> 2.32.0.3.g01195cf9f