Re: [RFC PATCH 6/9] virtio_net: construct multi-buffer xdp in mergeable

Jason Wang
2022-12-06T06:35:06Z

On Tue, Nov 22, 2022 at 3:44 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
> Build multi-buffer xdp using virtnet_build_xdp_buff() in mergeable.
>
> For the prefilled buffer before xdp is set, vq reset can be
> used to clear it, but most devices do not support it at present.
> In order not to bother users who are using xdp normally, we do
> not use vq reset for the time being.

I guess to tweak the part to say 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 | 67 +++++++++++++++++++++++-----------------
> 1 file changed, 38 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 20784b1d8236..83e6933ae62b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -994,6 +994,7 @@ 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);
> @@ -1024,53 +1025,50 @@ 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
> - * the receive path after XDP is loaded.
> - */
> - if (unlikely(hdr->hdr.gso_type))
> - goto err_xdp;

Two questions:

1) should we keep this check for the XDP program that can't deal with XDP frags?
2) how could we guarantee that the vnet header (gso_type/csum_start
etc) is still valid after XDP (where XDP program can choose to
override the header)?

> -
> - /* 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))) {
> - /* linearize data for XDP */
> - xdp_page = xdp_linearize_page(rq, &num_buf,
> - page, offset,
> - VIRTIO_XDP_HEADROOM,
> - &len);
> - frame_sz = PAGE_SIZE;
> + if (unlikely(headroom < virtnet_get_headroom(vi))) {
> + 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;

How can we know a single page is sufficient here? (before XDP is set,
we reserve neither headroom nor tailroom).

> offset = VIRTIO_XDP_HEADROOM;

I think we should still try to do linearization for the XDP program
that doesn't support XDP frags.

Thanks

> } 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(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++;
> @@ -1164,6 +1162,17 @@ 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();
>
> --
> 2.19.1.6.gb485710b
>