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

[Heng Qi <hengqi@linux.alibaba.com>]

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.

>> -
>> -               /* 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.

>
>>                          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
>>