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

Heng Qi
2022-12-08T08:30:10Z

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
>>