netdev - Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets when XDP is enabled

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Date:   Wed, 25 Jul 2018 13:22:13 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets
 when XDP is enabled

On 2018/07/25 4:10, Jakub Kicinski wrote:
> On Tue, 24 Jul 2018 18:39:09 +0900, Toshiaki Makita wrote:
>> On 2018/07/24 10:56, Toshiaki Makita wrote:
>>> On 2018/07/24 9:27, Jakub Kicinski wrote:  
>>>> On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:  
>>>>> From: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>>>>
>>>>> All oversized packets including GSO packets are dropped if XDP is
>>>>> enabled on receiver side, so don't send such packets from peer.
>>>>>
>>>>> Drop TSO and SCTP fragmentation features so that veth devices themselves
>>>>> segment packets with XDP enabled. Also cap MTU accordingly.
>>>>>
>>>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>  
>>>>
>>>> Is there any precedence for fixing up features and MTU like this?  Most
>>>> drivers just refuse to install the program if settings are incompatible.  
>>>
>>> I don't know any precedence. I can refuse the program on installing it
>>> when features and MTU are not appropriate. Is it preferred?
>>> Note that with current implementation wanted_features are not touched so
>>> features will be restored when the XDP program is removed. MTU will not
>>> be restored though, as I do not remember the original MTU.  
>>
>> I just recalled that virtio_net used to refused XDP when guest offload
>> features are incompatible but now it dynamically fixup them on
>> installing an XDP program.
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3f93522ffab2d46a36b57adf324a54e674fc9536
> 
> That's slightly different AFAIU, because the virtio features weren't
> really controllable at runtime at all.  I'm not dead set on leaving the
> features be, but I just want to make sure we think this through
> properly before we commit to any magic behaviour for ever...

To me it does not look so different. What the above virtio commit is
doing is almost disabling LRO so we could add a feature to toggle LRO
instead but it chose to automatically disable it. And this veth commit
is also almost equivalent to disabling LRO.
IMHO we should do this feature adjustment. It just avoids packet drops
and has no downside. It forces software segmentation on the peer veth.
Features will be restored when XDP is removed so there would be no
surprising for users. It seems there is no benefit to not doing this.

> Taking a quick glance at the MTU side, it seems that today if someone
> decides to set MTU on one side of a veth pair the packets will simply
> get dropped.  So the MTU coupling for XDP doesn't seem in line with
> existing behaviour of veth, not only other XDP drivers.

It looks weird to allow such inconsistent MTU settings. But anyway
changing MTU can have negative effect on users, such as causing
fragmentation or EMSGSIZE error on UDP sendmsg() or not restoring MTU. I
think I should not adjust MTU but just cap max_mtu.

-- 
Toshiaki Makita