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

Jason Wang
2022-12-13T07:10:09Z

On Thu, Dec 8, 2022 at 4:30 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
>
>
> 在 2022/12/6 下午2:33, Jason Wang 写道:
> > 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.
>
> OK, it works.
>
> >
> >> 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?
>
> Yes, the problem is the same as the xdp program without xdp.frags when
> GRO_HW, I will correct it.
>
> > 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)?
>
> We can save the vnet headr before the driver receives the packet and
> build xdp_buff, and then use
> the pre-saved value in the subsequent process.

The problem is that XDP may modify the packet (header) so some fields
are not valid any more (e.g csum_start/offset ?).

If I was not wrong, there's no way for the XDP program to access those
fields or does it support it right now?

>
> >> -
> >> - /* 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).
>
> This is only for the first buffer, refer to add_recvbuf_mergeable() and
> get_mergeable_buf_len() A buffer is always no larger than a page.

Ok.

Thanks

>
> >
> >> offset = VIRTIO_XDP_HEADROOM;
> > I think we should still try to do linearization for the XDP program
> > that doesn't support XDP frags.
>
> Yes, you are right.
>
> Thanks.
>
> >
> > 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
> >>
>