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

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Date:   Thu, 8 Dec 2022 16:20:00 +0800
From:   Heng Qi <hengqi@...ux.alibaba.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        "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>
Subject: Re: [RFC PATCH 1/9] virtio_net: disable the hole mechanism for xdp



在 2022/12/6 下午1:20, Jason Wang 写道:
> On Tue, Nov 22, 2022 at 3:44 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>> XDP core assumes that the frame_size of xdp_buff and the length of
>> the frag are PAGE_SIZE. But before xdp is set, the length of the prefilled
>> buffer may exceed PAGE_SIZE, which may cause the processing of xdp to fail,
>> so we disable the hole mechanism when xdp is loaded.
>>
>> 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..c5046d21b281 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 (!vi->xdp_enabled)
> How is this synchronized with virtnet_xdp_set()?
>
> I think we need to use headroom here since it did:
>
> static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
> {
>          return vi->xdp_enabled ? VIRTIO_XDP_HEADROOM : 0;
> }
>
> Otherwise xdp_enabled could be re-read which may lead bugs.

Yes, we should use headroom instead of using vi->xdp_enabled twice in 
the same
position to avoid re-reading.

Thanks for reminding.

>
> Thanks
>
>> +                       len += hole;
>>                  alloc_frag->offset += hole;
>>          }
>>
>> --
>> 2.19.1.6.gb485710b
>>