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

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Date:   Wed, 4 Jan 2017 10:57:19 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jason Wang <jasowang@...hat.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.

>>
>> As for capping it at GOOD_PACKET_LEN this has the nice benefit of avoiding any
>> underestimates in EWMA predictions because it appears min estimates are capped
>> at GOOD_PACKET_LEN via get_mergeable_buf_len().
> 
> This seems something misunderstanding here, I meant only use GOOD_PACKET_LEN for
> small buffer (which does not use EWMA).
> 

Yep I think its all cleared up now.

Thanks.