netdev - Re: [PATCH net-next 8/8] virtio_net: introduce receive_small

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Message-ID: <343825bad568ec0a21c283f876585585b040da9f.camel@redhat.com>
Date:   Thu, 30 Mar 2023 12:48:22 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "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-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 8/8] virtio_net: introduce receive_small_xdp()

On Tue, 2023-03-28 at 20:04 +0800, Xuan Zhuo wrote:
> @@ -949,15 +1042,11 @@ 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 header_offset = VIRTNET_RX_PAD;
>  	unsigned int headroom = vi->hdr_len + header_offset;

This changes (reduces) the headroom for non-xpd-pass skbs.

[...]
> +	buf += header_offset;
> +	memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);

AFAICS, that also means that receive_small(), for such packets, will
look for the virtio header in a different location. Is that expected?

Thanks.

Paolo