Re: [PATCH v3 5/9] virtio-net: construct multi-buffer xdp in mergeable

[Jason Wang <jasowang@redhat.com>]

在 2023/1/3 14:40, 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>


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

Thanks


> ---
>   drivers/net/virtio_net.c | 58 ++++++++++++++++++++++++++++++----------
>   1 file changed, 44 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 699e376b8f8b..ab01cf3855bc 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1036,7 +1036,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   	unsigned int metasize = 0;
>   	unsigned int tailroom = headroom ? sizeof(struct skb_shared_info) : 0;
>   	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
> -	unsigned int frame_sz;
> +	unsigned int frame_sz, xdp_room;
>   	int err;
>   
>   	head_skb = NULL;
> @@ -1057,11 +1057,14 @@ 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
> @@ -1077,14 +1080,16 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   		 */
>   		frame_sz = 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,
> @@ -1095,17 +1100,29 @@ 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))) {
> +			xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
> +						  sizeof(struct skb_shared_info));
> +			if (len + xdp_room > 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++;
> @@ -1201,6 +1218,19 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   				__free_pages(xdp_page, 0);
>   			goto err_xdp;
>   		}
> +err_xdp_frags:
> +		if (unlikely(xdp_page != page))
> +			__free_pages(xdp_page, 0);
> +
> +		if (xdp_buff_has_frags(&xdp)) {
> +			shinfo = xdp_get_shared_info_from_buff(&xdp);
> +			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();
>