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In-Reply-To: <a1f09866-a443-4f74-8025-6cdb32eb1d2c@daynix.com>
From:   Song Liu <song@kernel.org>
Date:   Tue, 21 Nov 2023 21:25:37 -0800
Message-ID: <CAPhsuW4o5o41a+jVjgGP+Ck3eUD8w6coLXMTYewXKJYmciLLnQ@mail.gmail.com>
Subject: Re: [RFC PATCH v2 1/7] bpf: Introduce BPF_PROG_TYPE_VNET_HASH
To:     Akihiko Odaki <akihiko.odaki@daynix.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Jason Wang <jasowang@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
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        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
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        "Michael S. Tsirkin" <mst@redhat.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
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        "open list:KERNEL SELFTEST FRAMEWORK" 
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On Mon, Nov 20, 2023 at 12:05=E2=80=AFAM Akihiko Odaki <akihiko.odaki@dayni=
x.com> wrote:
>
> On 2023/11/20 6:02, Song Liu wrote:
[...]
> >> In contrast, our intended use case is more like a normal application.
> >> So, for example, a user may download a container and run QEMU (includi=
ng
> >> the BPF program) installed in the container. As such, it is nice if th=
e
> >> ABI is stable across kernel releases, but it is not guaranteed for
> >> kfuncs. Such a use case is already covered with the eBPF steering
> >> program so I want to maintain it if possible.
> >
> > TBH, I don't think stability should be a concern for kfuncs used by QEM=
U.
> > Many core BPF APIs are now implemented as kfuncs: bpf_dynptr_*,
> > bpf_rcu_*, etc. As long as there are valid use cases,these kfuncs will
> > be supported.
>
> Documentation/bpf/kfuncs.rst still says:
>  > kfuncs provide a kernel <-> kernel API, and thus are not bound by any
>  > of the strict stability restrictions associated with kernel <-> user
>  > UAPIs.
>
> Is it possible to change the statement like as follows:
> "Most kfuncs provide a kernel <-> kernel API, and thus are not bound by
> any of the strict stability restrictions associated with kernel <-> user
> UAPIs. kfuncs that have same stability restrictions associated with
> UAPIs are exceptional, and must be carefully reviewed by subsystem (and
> BPF?) maintainers as any other UAPIs are."

I am afraid this is against the intention to not guarantee UAPI-level stabi=
lity
for kfuncs.

Thanks,
Song