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

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Date:   Tue, 10 Jan 2017 10:29:39 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        John Fastabend <john.fastabend@...il.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10日 07:58, Michael S. Tsirkin wrote:
> On Mon, Jan 09, 2017 at 03:49:27PM -0800, John Fastabend wrote:
>> On 17-01-09 03:24 PM, Michael S. Tsirkin wrote:
>>> On Mon, Jan 09, 2017 at 03:13:15PM -0800, John Fastabend wrote:
>>>> On 17-01-09 03:05 PM, Michael S. Tsirkin wrote:
>>>>> On Thu, Jan 05, 2017 at 11:09:14AM +0800, Jason Wang wrote:
>>>>>> 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
>>>>> Why does host care?
>>>>>
>>>> Well the guest will drop packets after mtu has been reduced.
>>> I didn't know. What place in code does this?
>>>
>> hmm in many of the drivers it is convention to use the mtu to set the rx
>> buffer sizes and a receive side max length filter. For example in the Intel
>> drivers if a packet with length greater than MTU + some headroom is received we
>> drop it. I guess in the networking stack RX path though nothing forces this and
>> virtio doesn't have any code to drop packets on rx size.
>>
>> In virtio I don't see any existing case currently. In the XDP case though we
>> need to ensure packets fit in a page for the time being which is why I was
>> looking at this code and generated this patch.
> I'd say just look at the hardware max mtu. Ignore the configured mtu.
>
>

Does this work for small buffers consider it always allocate skb with 
size of GOOD_PACKET_LEN? I think in any case, we should limit max_mtu to 
GOOD_PACKET_LEN for small buffers.

Thanks