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

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Date:   Wed, 11 Jan 2017 11:37:09 +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 13:00, Michael S. Tsirkin wrote:
> On Mon, Jan 09, 2017 at 08:25:43PM -0800, John Fastabend wrote:
>> On 17-01-09 07:55 PM, Michael S. Tsirkin wrote:
>>> On Mon, Jan 09, 2017 at 07:30:34PM -0800, John Fastabend wrote:
>>>> On 17-01-09 06:51 PM, Michael S. Tsirkin wrote:
>>>>> On Tue, Jan 10, 2017 at 10:29:39AM +0800, Jason Wang wrote:
>>>>>> 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?
>>>>> Spec says hardware won't send in packets > max mtu in config space.
>>>>>
>>>>>> I think in any case, we should limit max_mtu to
>>>>>> GOOD_PACKET_LEN for small buffers.
>>>>>>
>>>>>> Thanks
>>>>> XDP seems to have a bunch of weird restrictions, I just
>>>>> do not like it that the logic spills out to all drivers.
>>>>> What if someone decides to extend it to two pages in the future?
>>>>> Recode it all in all drivers ...
>>>>>
>>>>> Why can't net core enforce mtu?
>>>>>
>>>> OK I agree I'll put most the logic in rtnetlink.c when the program is added
>>>> or removed.
>>>>
>>>> But, I'm looking at the non-XDP receive_small path now and wondering how does
>>>> multiple buffer receives work (e.g. packet larger than GOOD_PACKET_LEN?)
>>> I don't understand the question. Look at add_recvbuf_small,
>>> it adds a tiny buffer for head and then the skb.
>>>
>> Specifically this seems to fail with mergeable buffers disabled
>>
>> On the host:
>>
>> # ip link set dev tap0 mtu 9000
>> # ping 22.2 -s 2048
>>
>> On the guest:
>>
>> # insmod ./drivers/net/virtio_net.ko
>> # ip link set dev eth0 mtu 9000
> Why would it work? You are sending a packet larger than ethernet MTU.

Ok, does it mean virtio-net does not support Jumbo frame? And if it 
can't work, use MAX_MTU as max_mtu is a bug to me.

>
>> With mergeable buffers enabled no problems it works as I expect at least.
> We don't expect to get these packets but
> mergeable is able to process them anyway.
> It's an accident:)  
>

But path MTU discovery indeed benefits from this "accident".

Thanks