netdev - Re: [PATCH v2 2/9] virtio_net: set up xdp for multi buffer packets

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Date:   Wed, 28 Dec 2022 14:27:35 +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 2/9] virtio_net: set up xdp for multi buffer packets


在 2022/12/28 11:50, Heng Qi 写道:
>
>
> 在 2022/12/27 下午8:20, Heng Qi 写道:
>>
>>
>> 在 2022/12/27 下午2:32, Jason Wang 写道:
>>>
>>> 在 2022/12/20 22:14, Heng Qi 写道:
>>>> When the xdp program sets xdp.frags, which means it can process
>>>> multi-buffer packets over larger MTU, so we continue to support xdp.
>>>> But for single-buffer xdp, we should keep checking for MTU.
>>>>
>>>> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
>>>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>>>> ---
>>>>   drivers/net/virtio_net.c | 4 ++--
>>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 443aa7b8f0ad..c5c4e9db4ed3 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -3095,8 +3095,8 @@ static int virtnet_xdp_set(struct net_device 
>>>> *dev, struct bpf_prog *prog,
>>>>           return -EINVAL;
>>>>       }
>>>>   -    if (dev->mtu > max_sz) {
>>>> -        NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP");
>>>> +    if (prog && !prog->aux->xdp_has_frags && dev->mtu > max_sz) {
>>>
>>>
>>> Not related to this patch, but I see:
>>>
>>>         unsigned long int max_sz = PAGE_SIZE - sizeof(struct 
>>> padded_vnet_hdr);
>>>
>>> Which is suspicious, do we need to count reserved headroom/tailroom 
>>> as well?
>>
>> This seems to be suspicious. After loading xdp, the size of the 
>> filled avail buffer
>> is (PAGE_SIZE - headroom - tailroom), so the size of the received 
>> used buffer, ie MTU,
>> should also be (PAGE_SIZE - headroom - tailroom).
>
> Hi Jason, this is indeed a problem. After verification, packet drop 
> will indeed occur.  To avoid this,
> the size of MTU should be (PAGE_SIZE - headroom - tailroom - ethhdr = 
> 4096 - 256 -320 - 14 =3506).
> Because when there is xdp, each filling is 3520 (PAGE_SIZE - room), if 
> the value of (MTU + 14) is
> greater than 3520 (because the MTU does not contain the ethernet 
> header), then the packet with a
> length greater than 3520 will come in, so num_buf will still be 
> greater than or equal to 2, and then
> xdp_linearize_page() will be performed and the packet will be dropped 
> because the total length is
> greater than PAGE_SIZE.
>
> I will make a separate bugfix patch to fix this later.


Great.

Thanks


>
> Thanks.
>
>>
>> Thanks.
>>
>>>
>>> Thanks
>>>
>>>
>>>> +        NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP 
>>>> without frags");
>>>>           netdev_warn(dev, "XDP requires MTU less than %lu\n", 
>>>> max_sz);
>>>>           return -EINVAL;
>>>>       }
>