netdev - Re: [net PATCH] net: virtio: cap mtu when XDP programs are running

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Message-ID: <068f0116-b37e-eb44-8c60-1781a9d5255e@redhat.com>
Date:   Thu, 5 Jan 2017 11:09:14 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>, mst@...hat.com
Cc:     john.r.fastabend@...el.com, netdev@...r.kernel.org,
        alexei.starovoitov@...il.com, daniel@...earbox.net
Subject: Re: [net PATCH] net: virtio: cap mtu when XDP programs are running



On 2017年01月05日 02:57, John Fastabend wrote:
> [...]
>
>> On 2017年01月04日 00:48, John Fastabend wrote:
>>> On 17-01-02 10:14 PM, Jason Wang wrote:
>>>> On 2017年01月03日 06:30, John Fastabend wrote:
>>>>> XDP programs can not consume multiple pages so we cap the MTU to
>>>>> avoid this case. Virtio-net however only checks the MTU at XDP
>>>>> program load and does not block MTU changes after the program
>>>>> has loaded.
>>>>>
>>>>> This patch sets/clears the max_mtu value at XDP load/unload time.
>>>>>
>>>>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
>>>>> ---
> [...]
>
>>> OK so this logic is a bit too simply. When it resets the max_mtu I guess it
>>> needs to read the mtu via
>>>
>>>      virtio_cread16(vdev, ...)
>>>
>>> or we may break the negotiated mtu.
>> Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to notify
>> the device about the mtu in this case which is not supported by virtio now.
> Note this is not really a XDP specific problem. The guest can change the MTU
> after init time even without XDP which I assume should ideally result in a
> notification if the MTU is negotiated.

Yes, Michael, do you think we need add some mechanism to notify host 
about MTU change in this case?

Thanks