netdev - Re: [PATCH v2 7/9] virtio_net: build skb from multi-buffer xdp

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Date:   Tue, 27 Dec 2022 15:51:59 +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 7/9] virtio_net: build skb from multi-buffer xdp



在 2022/12/27 下午3:31, Jason Wang 写道:
>
> 在 2022/12/20 22:14, Heng Qi 写道:
>> This converts the xdp_buff directly to a skb, including
>> multi-buffer and single buffer xdp. We'll isolate the
>> construction of skb based on xdp from page_to_skb().
>>
>> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>> ---
>>   drivers/net/virtio_net.c | 50 ++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 50 insertions(+)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 9f31bfa7f9a6..4e12196fcfd4 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -948,6 +948,56 @@ static struct sk_buff *receive_big(struct 
>> net_device *dev,
>>       return NULL;
>>   }
>>   +/* Why not use xdp_build_skb_from_frame() ?
>> + * XDP core assumes that xdp frags are PAGE_SIZE in length, while in
>> + * virtio-net there are 2 points that do not match its requirements:
>> + *  1. The size of the prefilled buffer is not fixed before xdp is set.
>> + *  2. When xdp is loaded, virtio-net has a hole mechanism (refer to
>> + *     add_recvbuf_mergeable()), which will make the size of a buffer
>> + *     exceed PAGE_SIZE.
>
>
> Is point 2 still valid after patch 1?

Yes, it is invalid anymore, I'll correct that, and there's a little more 
reason that
xdp_build_skb_from_frame() does more checks that we don't need, like
eth_type_trans() (which virtio-net does in receive_buf()).

>
> Other than this:
>
> Acked-by: Jason Wang <jasowang@...hat.com>

Thanks for your energy.

>
> Thanks
>
>
>> + */
>> +static struct sk_buff *build_skb_from_xdp_buff(struct net_device *dev,
>> +                           struct virtnet_info *vi,
>> +                           struct xdp_buff *xdp,
>> +                           unsigned int xdp_frags_truesz)
>> +{
>> +    struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>> +    unsigned int headroom, data_len;
>> +    struct sk_buff *skb;
>> +    int metasize;
>> +    u8 nr_frags;
>> +
>> +    if (unlikely(xdp->data_end > xdp_data_hard_end(xdp))) {
>> +        pr_debug("Error building skb as missing reserved tailroom 
>> for xdp");
>> +        return NULL;
>> +    }
>> +
>> +    if (unlikely(xdp_buff_has_frags(xdp)))
>> +        nr_frags = sinfo->nr_frags;
>> +
>> +    skb = build_skb(xdp->data_hard_start, xdp->frame_sz);
>> +    if (unlikely(!skb))
>> +        return NULL;
>> +
>> +    headroom = xdp->data - xdp->data_hard_start;
>> +    data_len = xdp->data_end - xdp->data;
>> +    skb_reserve(skb, headroom);
>> +    __skb_put(skb, data_len);
>> +
>> +    metasize = xdp->data - xdp->data_meta;
>> +    metasize = metasize > 0 ? metasize : 0;
>> +    if (metasize)
>> +        skb_metadata_set(skb, metasize);
>> +
>> +    if (unlikely(xdp_buff_has_frags(xdp)))
>> +        xdp_update_skb_shared_info(skb, nr_frags,
>> +                       sinfo->xdp_frags_size,
>> +                       xdp_frags_truesz,
>> +                       xdp_buff_is_frag_pfmemalloc(xdp));
>> +
>> +    return skb;
>> +}
>> +
>>   /* TODO: build xdp in big mode */
>>   static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
>>                         struct virtnet_info *vi,