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

Heng Qi
2022-12-16T09:42:37Z

在 2022/12/16 上午11:46, Jason Wang 写道:
> On Wed, Dec 14, 2022 at 4:38 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>> On Tue, Dec 13, 2022 at 03:08:46PM +0800, Jason Wang wrote:
>>> 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?
>>>
>> When guest_csum feature is negotiated, xdp cannot be set, because the metadata
>> of xdp_{buff, frame} may be adjusted by the bpf program, therefore,
>> csum_{start, offset} itself is invalid. And at the same time,
>> multi-buffer xdp programs should only Receive packets over larger MTU, so
>> we don't need gso related information anymore and need to disable GRO_HW.
> Ok, that's fine.
>
> (But it requires a large pMTU).

Yes. Like a jumbo frame that larger than 4K.

Thanks.

>
> Thanks
>
>> Thanks.
>>
>>>>>> -
>>>>>> - /* 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
>>>>>>