netdev - Re: [PATCH v2 1/9] virtio_net: disable the hole mechanism for xdp

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Date:   Tue, 27 Dec 2022 15:32:57 +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 1/9] virtio_net: disable the hole mechanism for xdp



在 2022/12/27 下午2:30, Jason Wang 写道:
>
> 在 2022/12/20 22:14, Heng Qi 写道:
>> XDP core assumes that the frame_size of xdp_buff and the length of
>> the frag are PAGE_SIZE. The hole may cause the processing of xdp to
>> fail, so we disable the hole mechanism when xdp is set.
>>
>> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>> ---
>>   drivers/net/virtio_net.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 9cce7dec7366..443aa7b8f0ad 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -1419,8 +1419,11 @@ static int add_recvbuf_mergeable(struct 
>> virtnet_info *vi,
>>           /* To avoid internal fragmentation, if there is very likely 
>> not
>>            * enough space for another buffer, add the remaining space to
>>            * the current buffer.
>> +         * XDP core assumes that frame_size of xdp_buff and the length
>> +         * of the frag are PAGE_SIZE, so we disable the hole mechanism.
>>            */
>> -        len += hole;
>> +        if (!headroom)
>> +            len += hole;
>
>
> Is this only a requirement of multi-buffer XDP? If not, it need to be 
> backported to stable.

It applies to single buffer xdp and multi-buffer xdp, but even if single 
buffer xdp has a hole
mechanism, there will be no problem (limiting mtu and turning off GUEST 
GSO), so there is
no need to backport it.

Thanks.

>
> Thanks
>
>
>>           alloc_frag->offset += hole;
>>       }