Date:   Wed, 28 Dec 2022 14:27:06 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Heng Qi <hengqi@...ux.alibaba.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 2022/12/27 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.

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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