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

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Date:   Wed, 28 Dec 2022 11:50:52 +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 2/9] virtio_net: set up xdp for multi buffer packets



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.

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;
>>>       }