netdev - Re: [xdp-hints] Re: [PATCH bpf-next v4 01/15] bpf: Document XDP RX metadata

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Message-ID: <87r0x1eegg.fsf@toke.dk>
Date:   Thu, 15 Dec 2022 00:46:23 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     David Vernet <void@...ifault.com>, bpf@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, 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
Subject: Re: [xdp-hints] Re: [PATCH bpf-next v4 01/15] bpf: Document XDP RX
 metadata

Stanislav Fomichev <sdf@...gle.com> writes:

> On Wed, Dec 14, 2022 at 2:34 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> Stanislav Fomichev <sdf@...gle.com> writes:
>>
>> > On Tue, Dec 13, 2022 at 8:37 AM David Vernet <void@...ifault.com> wrote:
>> >>
>> >> On Mon, Dec 12, 2022 at 06:35:51PM -0800, Stanislav Fomichev wrote:
>> >> > Document all current use-cases and assumptions.
>> >> >
>> >> > Cc: John Fastabend <john.fastabend@...il.com>
>> >> > Cc: David Ahern <dsahern@...il.com>
>> >> > Cc: Martin KaFai Lau <martin.lau@...ux.dev>
>> >> > Cc: Jakub Kicinski <kuba@...nel.org>
>> >> > Cc: Willem de Bruijn <willemb@...gle.com>
>> >> > Cc: Jesper Dangaard Brouer <brouer@...hat.com>
>> >> > Cc: Anatoly Burakov <anatoly.burakov@...el.com>
>> >> > Cc: Alexander Lobakin <alexandr.lobakin@...el.com>
>> >> > Cc: Magnus Karlsson <magnus.karlsson@...il.com>
>> >> > Cc: Maryam Tahhan <mtahhan@...hat.com>
>> >> > Cc: xdp-hints@...-project.net
>> >> > Cc: netdev@...r.kernel.org
>> >> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
>> >> > ---
>> >> >  Documentation/bpf/xdp-rx-metadata.rst | 90 +++++++++++++++++++++++++++
>> >> >  1 file changed, 90 insertions(+)
>> >> >  create mode 100644 Documentation/bpf/xdp-rx-metadata.rst
>> >> >
>> >> > diff --git a/Documentation/bpf/xdp-rx-metadata.rst b/Documentation/bpf/xdp-rx-metadata.rst
>> >> > new file mode 100644
>> >> > index 000000000000..498eae718275
>> >> > --- /dev/null
>> >> > +++ b/Documentation/bpf/xdp-rx-metadata.rst
>> >>
>> >> I think you need to add this to Documentation/bpf/index.rst. Or even
>> >> better, maybe it's time to add an xdp/ subdirectory and put all docs
>> >> there? Don't want to block your patchset from bikeshedding on this
>> >> point, so for now it's fine to just put it in
>> >> Documentation/bpf/index.rst until we figure that out.
>> >
>> > Maybe let's put it under Documentation/networking/xdp-rx-metadata.rst
>> > and reference form Documentation/networking/index.rst? Since it's more
>> > relevant to networking than the core bpf?
>> >
>> >> > @@ -0,0 +1,90 @@
>> >> > +===============
>> >> > +XDP RX Metadata
>> >> > +===============
>> >> > +
>> >> > +XDP programs support creating and passing custom metadata via
>> >> > +``bpf_xdp_adjust_meta``. This metadata can be consumed by the following
>> >> > +entities:
>> >>
>> >> Can you add a couple of sentences to this intro section that explains
>> >> what metadata is at a high level?
>> >
>> > I'm gonna copy-paste here what I'm adding, feel free to reply back if
>> > still unclear. (so we don't have to wait another week to discuss the
>> > changes)
>> >
>> > XDP programs support creating and passing custom metadata via
>> > ``bpf_xdp_adjust_meta``. The metadata can contain some extra information
>> > about the packet: timestamps, hash, vlan and tunneling information, etc.
>> > This metadata can be consumed by the following entities:
>>
>> This is not really accurate, though? The metadata area itself can
>> contain whatever the XDP program wants it to, and I think you're
>> conflating the "old" usage for arbitrary storage with the driver-kfunc
>> metadata support.
>>
>> I think we should clear separate the two: the metadata area is just a
>> place to store data (and is not consumed by the stack, except that
>> TC-BPF programs can access it), and the driver kfuncs are just a general
>> way to get data out of the drivers (and has nothing to do with the
>> metadata area, you can just get the data into stack variables).
>>
>> While it would be good to have a documentation of the general metadata
>> area stuff somewhere, I don't think it necessarily have to be part of
>> this series, so maybe just stick to documenting the kfuncs?
>
> Maybe I can reword to something like below?
>
> The metadata can be used to store some extra information about the
> packet timestamps, hash, vlan and tunneling information, etc.
>
> This way we are not actually defining what it is, but hinting about
> how it's commonly used?

Sent another reply to the original patch with some comments that are
hopefully a bit more constructive :)

-Toke