Re: [PATCH v2 5/9] virtio_net: construct multi-buffer xdp in mergeable

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

在 2022/12/27 下午3:01, Jason Wang 写道:
>
> 在 2022/12/20 22:14, Heng Qi 写道:
>> Build multi-buffer xdp using virtnet_build_xdp_buff_mrg().
>>
>> For the prefilled buffer before xdp is set, 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@linux.alibaba.com>
>> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>> ---
>>   drivers/net/virtio_net.c | 60 +++++++++++++++++++++++++++++-----------
>>   1 file changed, 44 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 8fc3b1841d92..40bc58fa57f5 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -1018,6 +1018,7 @@ 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);
>> @@ -1048,11 +1049,14 @@ 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
>> @@ -1061,19 +1065,23 @@ static struct sk_buff 
>> *receive_mergeable(struct net_device *dev,
>>           if (unlikely(hdr->hdr.gso_type))
>>               goto err_xdp;
>>   -        /* 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))) {
>> +        if (!xdp_prog->aux->xdp_has_frags &&
>> +            (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
>>               /* linearize data for XDP */
>>               xdp_page = xdp_linearize_page(rq, &num_buf,
>>                                 page, offset,
>> @@ -1084,17 +1092,26 @@ static struct sk_buff 
>> *receive_mergeable(struct net_device *dev,
>>               if (!xdp_page)
>>                   goto err_xdp;
>>               offset = VIRTIO_XDP_HEADROOM;
>> +        } else if (unlikely(headroom < virtnet_get_headroom(vi))) {
>
>
> I believe we need to check xdp_prog->aux->xdp_has_frags at least since 
> this may not work if it needs more than one frags?

Sorry Jason, I didn't understand you, I'll try to answer. For 
multi-buffer xdp programs, if the first buffer is a pre-filled buffer 
(no headroom),
we need to copy it out and use the subsequent buffers of this packet as 
its frags (this is done in virtnet_build_xdp_buff_mrg()), therefore,
it seems that there is no need to check 'xdp_prog->aux->xdp_has_frags' 
to mark multi-buffer xdp (of course I can add it),

+ } else if (unlikely(headroom < virtnet_get_headroom(vi))) {

Because the linearization of single-buffer xdp has all been done before, 
the subsequent situation can only be applied to multi-buffer xdp:
+ if (!xdp_prog->aux->xdp_has_frags &&
+ (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {

>
> Btw, I don't see a reason why we can't reuse xdp_linearize_page(), (we 
> probably don't need error is the buffer exceeds PAGE_SIZE).

For multi-buffer xdp, we only need to copy out the pre-filled first 
buffer, and use the remaining buffers of this packet as frags in 
virtnet_build_xdp_buff_mrg().

Thanks.

>
> Other looks good.
>
> Thanks
>
>
>> +            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;
>> +            offset = VIRTIO_XDP_HEADROOM;
>>           } 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_mrg(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++;
>> @@ -1190,6 +1207,17 @@ 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();