linux-kernel - Re: [PATCH bpf-next v3] bpf/docs: Document kfunc lifecycle / stability expectations

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Date:   Fri, 3 Feb 2023 09:03:50 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...a.com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...a.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        bagasdotme@...il.com, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3] bpf/docs: Document kfunc lifecycle /
 stability expectations

On Fri, Feb 3, 2023 at 7:57 AM David Vernet <void@...ifault.com> wrote:
>
> BPF kernel <-> kernel API stability has been discussed at length over
> the last several weeks and months. Now that we've largely aligned over
> kfuncs being the way forward, and BPF helpers being considered
> functionally frozen, it's time to document the expectations for kfunc
> lifecycles and stability so that everyone (BPF users, kfunc developers,
> and maintainers) are all aligned, and have a crystal-clear understanding
> of the expectations surrounding kfuncs.
>
> To do that, this patch adds that documentation to the main kfuncs
> documentation page via a new 'kfunc lifecycle expectations' section. The
> patch describes how decisions are made in the kernel regarding whether
> to include, keep, deprecate, or change / remove a kfunc. As described
> very overtly in the patch itself, but likely worth highlighting here:
>
> "kfunc stability" does not mean, nor ever will mean, "BPF APIs may block
> development elsewhere in the kernel".
>
> Rather, the intention and expectation is for kfuncs to be treated like
> EXPORT_SYMBOL_GPL symbols in the kernel. The goal is for kfuncs to be a
> safe and valuable option for maintainers and kfunc developers to extend
> the kernel, without tying anyone's hands, or imposing any kind of
> restrictions on maintainers in the same way that UAPI changes do.
>
> In addition to the 'kfunc lifecycle expectations' section, this patch
> also adds documentation for a new KF_DEPRECATED kfunc flag which kfunc
> authors or maintainers can choose to add to kfuncs if and when they
> decide to deprecate them. Note that as described in the patch itself, a
> kfunc need not be deprecated before being changed or removed -- this
> flag is simply provided as an available deprecation mechanism for those
> that want to provide a deprecation story / timeline to their users.
> When necessary, kfuncs may be changed or removed to accommodate changes
> elsewhere in the kernel without any deprecation at all.
>
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Co-developed-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: David Vernet <void@...ifault.com>

David, Toke,
Thanks a lot for writing it down.
It certainly captures the main points.
Applied.