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

Jason Wang
2022-12-27T07:02:54Z

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();
>