netdev - Re: Does skb_metadata_differs really need to stop GRO aggregation?

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Message-ID: <8677db3e-5662-7ebe-5af0-e5a3ca60587f@iogearbox.net>
Date: Thu, 30 Nov 2023 17:32:23 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Edward Cree <ecree.xilinx@...il.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>
Cc: Yan Zhai <yan@...udflare.com>, Stanislav Fomichev <sdf@...gle.com>,
 Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
 Alexei Starovoitov <ast@...nel.org>, kernel-team
 <kernel-team@...udflare.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
 "David S. Miller" <davem@...emloft.net>,
 Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: Does skb_metadata_differs really need to stop GRO aggregation?

On 11/30/23 2:55 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@...earbox.net> writes
>> On 11/29/23 10:52 PM, Toke Høiland-Jørgensen wrote:
>>> Edward Cree <ecree.xilinx@...il.com> writes:
>>>> On 28/11/2023 14:39, Toke Høiland-Jørgensen wrote:
>>>>> I'm not quite sure what should be the semantics of that, though. I.e.,
>>>>> if you are trying to aggregate two packets that have the flag set, which
>>>>> packet do you take the value from? What if only one packet has the flag
>>
>> It would probably make sense if both packets have it set.
> 
> Right, so "aggregate only if both packets have the flag set, keeping the
> metadata area from the first packet", then?

Yes, sgtm.

>>>>> set? Or should we instead have a "metadata_xdp_only" flag that just
>>>>> prevents the skb metadata field from being set entirely?
>>
>> What would be the use case compared to resetting meta data right before
>> we return with XDP_PASS?
> 
> I was thinking it could save a call to xdp_adjust_meta() to reset it
> back to zero before PASSing the packet. But okay, that may be of
> marginal utility.

Agree, feels too marginal.

>>>> Sounds like what's actually needed is bpf progs inside the GRO engine
>>>>    to implement the metadata "protocol" prepare and coalesce callbacks?
>>>
>>> Hmm, yes, I guess that would be the most general solution :)
>>
>> Feels like a potential good fit, agree, although for just solving the
>> above sth not requiring extra BPF might be nice as well.
> 
> Yeah, I agree that just the flag makes sense on its own.
> 
> -Toke
>