Re: [PATCH v2 5/9] virtio_net: construct multi-buffer xdp in mergeable

[Jason Wang <jasowang@redhat.com>]

在 2022/12/20 22:14, Heng Qi 写道:
> Build multi-buffer xdp using virtnet_build_xdp_buff_mrg().
>
> For the prefilled buffer before xdp is set, we will probably use
> vq reset in the future. At the same time, virtio net currently
> uses comp pages, and bpf_xdp_frags_increase_tail() needs to calculate
> the tailroom of the last frag, which will involve the offset of the
> corresponding page and cause a negative value, so we disable tail
> increase by not setting xdp_rxq->frag_size.
>
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 60 +++++++++++++++++++++++++++++-----------
>   1 file changed, 44 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 8fc3b1841d92..40bc58fa57f5 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1018,6 +1018,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   					 unsigned int *xdp_xmit,
>   					 struct virtnet_rq_stats *stats)
>   {
> +	unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>   	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
>   	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
>   	struct page *page = virt_to_head_page(buf);
> @@ -1048,11 +1049,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   	rcu_read_lock();
>   	xdp_prog = rcu_dereference(rq->xdp_prog);
>   	if (xdp_prog) {
> +		unsigned int xdp_frags_truesz = 0;
> +		struct skb_shared_info *shinfo;
>   		struct xdp_frame *xdpf;
>   		struct page *xdp_page;
>   		struct xdp_buff xdp;
>   		void *data;
>   		u32 act;
> +		int i;
>   
>   		/* Transient failure which in theory could occur if
>   		 * in-flight packets from before XDP was enabled reach
> @@ -1061,19 +1065,23 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   		if (unlikely(hdr->hdr.gso_type))
>   			goto err_xdp;
>   
> -		/* Buffers with headroom use PAGE_SIZE as alloc size,
> -		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> +		/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
> +		 * with headroom may add hole in truesize, which
> +		 * make their length exceed PAGE_SIZE. So we disabled the
> +		 * hole mechanism for xdp. See add_recvbuf_mergeable().
>   		 */
>   		frame_sz = headroom ? PAGE_SIZE : truesize;
>   
> -		/* This happens when rx buffer size is underestimated
> -		 * or headroom is not enough because of the buffer
> -		 * was refilled before XDP is set. This should only
> -		 * happen for the first several packets, so we don't
> -		 * care much about its performance.
> +		/* This happens when headroom is not enough because
> +		 * of the buffer was prefilled before XDP is set.
> +		 * This should only happen for the first several packets.
> +		 * In fact, vq reset can be used here to help us clean up
> +		 * the prefilled buffers, but many existing devices do not
> +		 * support it, and we don't want to bother users who are
> +		 * using xdp normally.
>   		 */
> -		if (unlikely(num_buf > 1 ||
> -			     headroom < virtnet_get_headroom(vi))) {
> +		if (!xdp_prog->aux->xdp_has_frags &&
> +		    (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
>   			/* linearize data for XDP */
>   			xdp_page = xdp_linearize_page(rq, &num_buf,
>   						      page, offset,
> @@ -1084,17 +1092,26 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   			if (!xdp_page)
>   				goto err_xdp;
>   			offset = VIRTIO_XDP_HEADROOM;
> +		} else if (unlikely(headroom < virtnet_get_headroom(vi))) {


I believe we need to check xdp_prog->aux->xdp_has_frags at least since 
this may not work if it needs more than one frags?

Btw, I don't see a reason why we can't reuse xdp_linearize_page(), (we 
probably don't need error is the buffer exceeds PAGE_SIZE).

Other looks good.

Thanks


> +			if ((VIRTIO_XDP_HEADROOM + len + tailroom) > PAGE_SIZE)
> +				goto err_xdp;
> +
> +			xdp_page = alloc_page(GFP_ATOMIC);
> +			if (!xdp_page)
> +				goto err_xdp;
> +
> +			memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
> +			       page_address(page) + offset, len);
> +			frame_sz = PAGE_SIZE;
> +			offset = VIRTIO_XDP_HEADROOM;
>   		} else {
>   			xdp_page = page;
>   		}
> -
> -		/* Allow consuming headroom but reserve enough space to push
> -		 * the descriptor on if we get an XDP_TX return code.
> -		 */
>   		data = page_address(xdp_page) + offset;
> -		xdp_init_buff(&xdp, frame_sz - vi->hdr_len, &rq->xdp_rxq);
> -		xdp_prepare_buff(&xdp, data - VIRTIO_XDP_HEADROOM + vi->hdr_len,
> -				 VIRTIO_XDP_HEADROOM, len - vi->hdr_len, true);
> +		err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, len, frame_sz,
> +						 &num_buf, &xdp_frags_truesz, stats);
> +		if (unlikely(err))
> +			goto err_xdp_frags;
>   
>   		act = bpf_prog_run_xdp(xdp_prog, &xdp);
>   		stats->xdp_packets++;
> @@ -1190,6 +1207,17 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   				__free_pages(xdp_page, 0);
>   			goto err_xdp;
>   		}
> +err_xdp_frags:
> +		shinfo = xdp_get_shared_info_from_buff(&xdp);
> +
> +		if (unlikely(xdp_page != page))
> +			__free_pages(xdp_page, 0);
> +
> +		for (i = 0; i < shinfo->nr_frags; i++) {
> +			xdp_page = skb_frag_page(&shinfo->frags[i]);
> +			put_page(xdp_page);
> +		}
> +		goto err_xdp;
>   	}
>   	rcu_read_unlock();
>