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

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Message-ID: <Y+AUm8cikB6m978j@pop-os.localdomain>
Date:   Sun, 5 Feb 2023 12:42:03 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...a.com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com,
        toke@...hat.com, corbet@....net, linux-doc@...r.kernel.org,
        brouer@...hat.com, bagasdotme@...il.com, linux-api@...r.kernel.org
Subject: Re: [PATCH bpf-next v3] bpf/docs: Document kfunc lifecycle /
 stability expectations

On Fri, Feb 03, 2023 at 09:57:27AM -0600, David Vernet 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.

I think they are still different, kernel modules are still considered as
a part of kernel development, while eBPF code is not that supposed to be
kernel development, at least much further. Treating them alike is
misleading, IMHO.

> 
> 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.

This fundamentally contradicts with Compile-Once-Run-Everywhere
https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html
Could you add some clarification for this too? Especically how we could
respect CO-RE meanwhile deprecating kfuncs?

BTW, not related to compatibility, but still kfuncs related confusion,
it also contradicts with Documentation/bpf/bpf_design_QA.rst:

"
Q: Can BPF functionality such as new program or map types, new
helpers, etc be added out of kernel module code?

A: NO.
"

The conntrack kfuncs like bpf_skb_ct_alloc() reside in a kernel module.

Thanks!