netdev - Re: [PATCH v2 7/9] virtio_net: build skb from multi-buffer xdp

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Date:   Tue, 27 Dec 2022 15:31:41 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Heng Qi <hengqi@...ux.alibaba.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH v2 7/9] virtio_net: build skb from multi-buffer xdp


在 2022/12/20 22:14, Heng Qi 写道:
> This converts the xdp_buff directly to a skb, including
> multi-buffer and single buffer xdp. We'll isolate the
> construction of skb based on xdp from page_to_skb().
>
> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 50 ++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 50 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9f31bfa7f9a6..4e12196fcfd4 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -948,6 +948,56 @@ static struct sk_buff *receive_big(struct net_device *dev,
>   	return NULL;
>   }
>   
> +/* Why not use xdp_build_skb_from_frame() ?
> + * XDP core assumes that xdp frags are PAGE_SIZE in length, while in
> + * virtio-net there are 2 points that do not match its requirements:
> + *  1. The size of the prefilled buffer is not fixed before xdp is set.
> + *  2. When xdp is loaded, virtio-net has a hole mechanism (refer to
> + *     add_recvbuf_mergeable()), which will make the size of a buffer
> + *     exceed PAGE_SIZE.


Is point 2 still valid after patch 1?

Other than this:

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks


> + */
> +static struct sk_buff *build_skb_from_xdp_buff(struct net_device *dev,
> +					       struct virtnet_info *vi,
> +					       struct xdp_buff *xdp,
> +					       unsigned int xdp_frags_truesz)
> +{
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	unsigned int headroom, data_len;
> +	struct sk_buff *skb;
> +	int metasize;
> +	u8 nr_frags;
> +
> +	if (unlikely(xdp->data_end > xdp_data_hard_end(xdp))) {
> +		pr_debug("Error building skb as missing reserved tailroom for xdp");
> +		return NULL;
> +	}
> +
> +	if (unlikely(xdp_buff_has_frags(xdp)))
> +		nr_frags = sinfo->nr_frags;
> +
> +	skb = build_skb(xdp->data_hard_start, xdp->frame_sz);
> +	if (unlikely(!skb))
> +		return NULL;
> +
> +	headroom = xdp->data - xdp->data_hard_start;
> +	data_len = xdp->data_end - xdp->data;
> +	skb_reserve(skb, headroom);
> +	__skb_put(skb, data_len);
> +
> +	metasize = xdp->data - xdp->data_meta;
> +	metasize = metasize > 0 ? metasize : 0;
> +	if (metasize)
> +		skb_metadata_set(skb, metasize);
> +
> +	if (unlikely(xdp_buff_has_frags(xdp)))
> +		xdp_update_skb_shared_info(skb, nr_frags,
> +					   sinfo->xdp_frags_size,
> +					   xdp_frags_truesz,
> +					   xdp_buff_is_frag_pfmemalloc(xdp));
> +
> +	return skb;
> +}
> +
>   /* TODO: build xdp in big mode */
>   static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
>   				      struct virtnet_info *vi,