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

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Message-ID: <2704f3ad-9477-c9d2-8eca-01a9fa92732a@linux.alibaba.com>
Date:   Tue, 27 Dec 2022 20:20:01 +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).

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;
>>       }