Date:   Wed, 28 Dec 2022 16:17:40 +0800
From:   Heng Qi <hengqi@...ux.alibaba.com>
To:     Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH v2 4/9] virtio_net: build xdp_buff with multi buffers



在 2022/12/28 下午2:27, Jason Wang 写道:
>
> 在 2022/12/27 17:10, Heng Qi 写道:
>>
>>
>> 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@...ux.alibaba.com>
>>>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.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?
>
>
> The problem is:
>
> We had
>
> static unsigned int mergeable_ctx_to_truesize(void *mrg_ctx)
>
> It means the truesize should be calculated before packing into the 
> context. So having another math to get truesize is very confusing, I 
> think it's better to fix that. Otherwise we may end up code that is 
> hard to be reviewed even with the help of comments.

It is reasonable to let truesize return to its literal meaning, which 
includes the length of
headroom and tailroom, and I will modify this in the next version.

Thanks.

>
> Thanks
>
>
>> 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,
>>

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