netdev - Re: [PATCH v2 4/9] virtio_net: build xdp

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Date:   Tue, 27 Dec 2022 14:46:23 +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 4/9] virtio_net: build xdp_buff with multi buffers


在 2022/12/20 22:14, Heng Qi 写道:
> Support xdp for multi buffer packets in mergeable mode.
>
> Putting the first buffer as the linear part for xdp_buff,
> and the rest of the buffers as non-linear fragments to struct
> skb_shared_info in the tailroom belonging to xdp_buff.
>
> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 78 ++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 78 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 08f209d7b0bf..8fc3b1841d92 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -931,6 +931,84 @@ static struct sk_buff *receive_big(struct net_device *dev,
>   	return NULL;
>   }
>   
> +/* TODO: build xdp in big mode */
> +static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> +				      struct virtnet_info *vi,
> +				      struct receive_queue *rq,
> +				      struct xdp_buff *xdp,
> +				      void *buf,
> +				      unsigned int len,
> +				      unsigned int frame_sz,
> +				      u16 *num_buf,
> +				      unsigned int *xdp_frags_truesize,
> +				      struct virtnet_rq_stats *stats)
> +{
> +	unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> +	unsigned int truesize, headroom;
> +	struct skb_shared_info *shinfo;
> +	unsigned int xdp_frags_truesz = 0;
> +	unsigned int cur_frag_size;
> +	struct page *page;
> +	skb_frag_t *frag;
> +	int offset;
> +	void *ctx;
> +
> +	xdp_init_buff(xdp, frame_sz, &rq->xdp_rxq);
> +	xdp_prepare_buff(xdp, buf - VIRTIO_XDP_HEADROOM,
> +			 VIRTIO_XDP_HEADROOM + vi->hdr_len, len - vi->hdr_len, true);
> +
> +	if (*num_buf > 1) {
> +		shinfo = xdp_get_shared_info_from_buff(xdp);
> +		shinfo->nr_frags = 0;
> +		shinfo->xdp_frags_size = 0;
> +	}
> +
> +	if ((*num_buf - 1) > MAX_SKB_FRAGS)
> +		return -EINVAL;
> +
> +	while ((--*num_buf) >= 1) {
> +		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> +		if (unlikely(!buf)) {
> +			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> +				 dev->name, *num_buf,
> +				 virtio16_to_cpu(vi->vdev, hdr->num_buffers));
> +			dev->stats.rx_length_errors++;
> +			return -EINVAL;
> +		}
> +
> +		if (!xdp_buff_has_frags(xdp))
> +			xdp_buff_set_frags_flag(xdp);


Any reason to put this inside the loop?


> +
> +		stats->bytes += len;
> +		page = virt_to_head_page(buf);
> +		offset = buf - page_address(page);
> +		truesize = mergeable_ctx_to_truesize(ctx);
> +		headroom = mergeable_ctx_to_headroom(ctx);
> +
> +		cur_frag_size = truesize + (headroom ? (headroom + tailroom) : 0);
> +		xdp_frags_truesz += cur_frag_size;


Not related to this patch, but it would easily confuse the future 
readers that the we need another math for truesize. I think at least we 
need some comments for this or

I guess the root cause is in get_mergeable_buf_len:

static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
                                       struct ewma_pkt_len *avg_pkt_len,
                                           unsigned int room)
{
         struct virtnet_info *vi = rq->vq->vdev->priv;
         const size_t hdr_len = vi->hdr_len;
         unsigned int len;

         if (room)
         return PAGE_SIZE - room;

And we do

     len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);

     ...

     ctx = mergeable_len_to_ctx(len, headroom);


I wonder if it's better to pack the real truesize (PAGE_SIZE) here. This 
may ease a lot of things.

Thanks


> +		if (unlikely(len > truesize || cur_frag_size > PAGE_SIZE)) {
> +			pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
> +				 dev->name, len, (unsigned long)ctx);
> +			dev->stats.rx_length_errors++;
> +			return -EINVAL;
> +		}
> +
> +		frag = &shinfo->frags[shinfo->nr_frags++];
> +		__skb_frag_set_page(frag, page);
> +		skb_frag_off_set(frag, offset);
> +		skb_frag_size_set(frag, len);
> +		if (page_is_pfmemalloc(page))
> +			xdp_buff_set_frag_pfmemalloc(xdp);
> +
> +		shinfo->xdp_frags_size += len;
> +	}
> +
> +	*xdp_frags_truesize = xdp_frags_truesz;
> +	return 0;
> +}
> +
>   static struct sk_buff *receive_mergeable(struct net_device *dev,
>   					 struct virtnet_info *vi,
>   					 struct receive_queue *rq,