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

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On Tue, Nov 22, 2022 at 3:44 PM Heng Qi <hengqi@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> ---
>  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
>




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