netdev - Re: [xdp-hints] Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context

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Message-ID: <87iljl7rl1.fsf@toke.dk>
Date:   Fri, 11 Nov 2022 10:44:58 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Martin KaFai Lau <martin.lau@...ux.dev>
Cc:     Stanislav Fomichev <sdf@...gle.com>, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        haoluo@...gle.com, jolsa@...nel.org,
        David Ahern <dsahern@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [xdp-hints] Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp
 metadata into skb context

Martin KaFai Lau <martin.lau@...ux.dev> writes:

> On 11/10/22 3:29 PM, Toke Høiland-Jørgensen wrote:
>>>> For the metadata consumed by the stack right now it's a bit
>>>> hypothetical, yeah. However, there's a bunch of metadata commonly
>>>> supported by hardware that the stack currently doesn't consume and that
>>>> hopefully this feature will end up making more accessible. My hope is
>>>> that the stack can also learn how to use this in the future, in which
>>>> case we may run out of space. So I think of that bit mostly as
>>>> future-proofing...
>>>
>>> ic. in this case, Can the btf_id be added to 'struct xdp_to_skb_metadata' later
>>> if it is indeed needed?  The 'struct xdp_to_skb_metadata' is not in UAPI and
>>> doing it with CO-RE is to give us flexibility to make this kind of changes in
>>> the future.
>> 
>> My worry is mostly that it'll be more painful to add it later than just
>> including it from the start, mostly because of AF_XDP users. But if we
>> do the randomisation thing (thus forcing AF_XDP users to deal with the
>> dynamic layout as well), it should be possible to add it later, and I
>> can live with that option as well...
>
> imo, considering we are trying to optimize unnecessary field
> initialization as below, it is sort of wasteful to always initialize
> the btf_id with the same value. It is better to add it in the future
> when there is a need.

Okay, let's omit the BTF ID for now, and see what that looks like. I'll
try to keep in mind to see if I can find any reasons why we'd need to
add it back and make sure to complain before this lands if I find any :)

-Toke