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

Alexei Starovoitov
2023-02-03T17:04:07Z

On Fri, Feb 3, 2023 at 7:57 AM David Vernet <void@manifault.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@gmail.com>
> Co-developed-by: Toke Høiland-Jørgensen <toke@redhat.com>
> Signed-off-by: David Vernet <void@manifault.com>

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