Re: [PATCH v2 4/9] virtio_net: build xdp_buff with multi buffers

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

在 2022/12/27 下午2:46, Jason Wang 写道:
>
> 在 2022/12/20 22:14, Heng Qi 写道:
>> Support xdp for multi buffer packets in mergeable mode.
>>
>> Putting the first buffer as the linear part for xdp_buff,
>> and the rest of the buffers as non-linear fragments to struct
>> skb_shared_info in the tailroom belonging to xdp_buff.
>>
>> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
>> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>> ---
>>   drivers/net/virtio_net.c | 78 ++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 78 insertions(+)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 08f209d7b0bf..8fc3b1841d92 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -931,6 +931,84 @@ static struct sk_buff *receive_big(struct 
>> net_device *dev,
>>       return NULL;
>>   }
>>   +/* TODO: build xdp in big mode */
>> +static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
>> +                      struct virtnet_info *vi,
>> +                      struct receive_queue *rq,
>> +                      struct xdp_buff *xdp,
>> +                      void *buf,
>> +                      unsigned int len,
>> +                      unsigned int frame_sz,
>> +                      u16 *num_buf,
>> +                      unsigned int *xdp_frags_truesize,
>> +                      struct virtnet_rq_stats *stats)
>> +{
>> +    unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct 
>> skb_shared_info));
>> +    struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
>> +    unsigned int truesize, headroom;
>> +    struct skb_shared_info *shinfo;
>> +    unsigned int xdp_frags_truesz = 0;
>> +    unsigned int cur_frag_size;
>> +    struct page *page;
>> +    skb_frag_t *frag;
>> +    int offset;
>> +    void *ctx;
>> +
>> +    xdp_init_buff(xdp, frame_sz, &rq->xdp_rxq);
>> +    xdp_prepare_buff(xdp, buf - VIRTIO_XDP_HEADROOM,
>> +             VIRTIO_XDP_HEADROOM + vi->hdr_len, len - vi->hdr_len, 
>> true);
>> +
>> +    if (*num_buf > 1) {
>> +        shinfo = xdp_get_shared_info_from_buff(xdp);
>> +        shinfo->nr_frags = 0;
>> +        shinfo->xdp_frags_size = 0;
>> +    }
>> +
>> +    if ((*num_buf - 1) > MAX_SKB_FRAGS)
>> +        return -EINVAL;
>> +
>> +    while ((--*num_buf) >= 1) {
>> +        buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
>> +        if (unlikely(!buf)) {
>> +            pr_debug("%s: rx error: %d buffers out of %d missing\n",
>> +                 dev->name, *num_buf,
>> +                 virtio16_to_cpu(vi->vdev, hdr->num_buffers));
>> +            dev->stats.rx_length_errors++;
>> +            return -EINVAL;
>> +        }
>> +
>> +        if (!xdp_buff_has_frags(xdp))
>> +            xdp_buff_set_frags_flag(xdp);
>
>
> Any reason to put this inside the loop?

I'll move it outside the loop in the next version.

>
>
>> +
>> +        stats->bytes += len;
>> +        page = virt_to_head_page(buf);
>> +        offset = buf - page_address(page);
>> +        truesize = mergeable_ctx_to_truesize(ctx);
>> +        headroom = mergeable_ctx_to_headroom(ctx);
>> +
>> +        cur_frag_size = truesize + (headroom ? (headroom + tailroom) 
>> : 0);
>> +        xdp_frags_truesz += cur_frag_size;
>
>
> Not related to this patch, but it would easily confuse the future 
> readers that the we need another math for truesize. I think at least 
> we need some comments for this or

Yes it might, I'll add more comments on this.

>
> I guess the root cause is in get_mergeable_buf_len:
>
> static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
>                                       struct ewma_pkt_len *avg_pkt_len,
>                                           unsigned int room)
> {
>         struct virtnet_info *vi = rq->vq->vdev->priv;
>         const size_t hdr_len = vi->hdr_len;
>         unsigned int len;
>
>         if (room)
>         return PAGE_SIZE - room;
>
> And we do
>
>     len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);
>
>     ...
>
>     ctx = mergeable_len_to_ctx(len, headroom);
>
>
> I wonder if it's better to pack the real truesize (PAGE_SIZE) here. 
> This may ease a lot of things.

I don't know the historical reason for not packing, but I guess this is 
for the convenience of
comparing the actual length len given by the device with truesize. 
Therefore, I think it would
be better to keep the above practice for now? Perhaps, I can add more 
explanation to the
above code to help future readers try not to get confused.

Thanks.

>
> Thanks
>
>
>> +        if (unlikely(len > truesize || cur_frag_size > PAGE_SIZE)) {
>> +            pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
>> +                 dev->name, len, (unsigned long)ctx);
>> +            dev->stats.rx_length_errors++;
>> +            return -EINVAL;
>> +        }
>> +
>> +        frag = &shinfo->frags[shinfo->nr_frags++];
>> +        __skb_frag_set_page(frag, page);
>> +        skb_frag_off_set(frag, offset);
>> +        skb_frag_size_set(frag, len);
>> +        if (page_is_pfmemalloc(page))
>> +            xdp_buff_set_frag_pfmemalloc(xdp);
>> +
>> +        shinfo->xdp_frags_size += len;
>> +    }
>> +
>> +    *xdp_frags_truesize = xdp_frags_truesz;
>> +    return 0;
>> +}
>> +
>>   static struct sk_buff *receive_mergeable(struct net_device *dev,
>>                        struct virtnet_info *vi,
>>                        struct receive_queue *rq,