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From:   Song Liu <song@kernel.org>
Date:   Mon, 11 Dec 2023 09:40:10 -0800
Message-ID: <CAPhsuW7E6Dyp8VTiuwbreB18Lt93n9Fa2hdd2sEschUDzgQiSg@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>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
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On Sun, Dec 10, 2023 at 9:04=E2=80=AFPM Akihiko Odaki <akihiko.odaki@daynix=
.com> wrote:
>
[...]
> >
> > I don't think we can provide stability guarantees before seeing somethi=
ng
> > being used in the field. How do we know it will be useful forever? If a
> > couple years later, there is only one person using it somewhere in the
> > world, why should we keep supporting it? If there are millions of virtu=
al
> > machines using it, why would you worry about it being removed?
>
> I have a different opinion about providing stability guarantees; I
> believe it is safe to provide such a guarantee without actual use in a
> field. We develop features expecting there are real uses, and if it
> turns out otherwise, we can break the stated guarantee since there is no
> real use cases anyway. It is fine even breaking UAPIs in such a case,
> which is stated in Documentation/admin-guide/reporting-regressions.rst.
>
> So I rather feel easy about guaranteeing UAPI stability; we can just
> guarantee the UAPI-level stability for a particular kfunc and use it
> from QEMU expecting the stability. If the feature is found not useful,
> QEMU and the kernel can just remove it.

It appears that we more or less agree that this feature may not be
something we will support forever.

> I'm more concerned about the other case, which means that there will be
> wide uses of this feature. A kernel developer may assume the stability
> of the interface is like one of kernel internal APIs
> (Documentation/bpf/kfuncs.rst says kfuncs are like EXPORT_SYMBOL_GPL)
> and decide to change it, breaking old QEMU binaries and that's something
> I would like to avoid.
>
> Regarding the breakage scenario, I think we can avoid the kfuncs removal
> just by saying "we won't remove them". I'm more worried the case that a
> change in the BPF kfunc infrastucture requires to recompile the binary.
>
> So, in short, I don't think we can say "kfuncs are like
> EXPORT_SYMBOL_GPL" and "you can freely use kfuncs in a normal userspace
> application like QEMU" at the same time.
>
> >
[...]
>
> >
> > I would recommend you give option 2 a try and share the code. This is
> > probably the best way to move the discussion forward.
>
> I'd like to add a documentation change to say the added kfuncs are
> exceptional cases that are not like EXPORT_SYMBOL_GPL in that case. Will
> it work?

It will not.

The BPF community had a lot of discussions about the stability of BPF APIs,=
 for
example in [1]. Therefore, this is not a light decision.

AFAICT, what is being proposed in this RFC is way less stable than many kfu=
ncs
we added recently. We are not changing the stability guarantee for this. Le=
t's
invest our time wisely and work on more meaningful things, for example, a
prototype that may actually get accepted.

Thanks,
Song

[1] https://lore.kernel.org/bpf/20221207205537.860248-1-joannelkoong@gmail.=
com/