Re: [PATCH dwarves v2] btf: Add support for the floating-point types

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	dwarves@vger.kernel.org, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Yonghong Song <yhs@fb.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [PATCH dwarves v2] btf: Add support for the floating-point types
Date: Wed, 10 Mar 2021 10:37:01 -0300	[thread overview]
Message-ID: <YEjLfbBQSxiWQMLD@kernel.org> (raw)
In-Reply-To: <CAEf4BzZo4DJJgB57wrkDZCzBGf876ixZBjQrJE4XM_y7OTDKKQ@mail.gmail.com>

Em Tue, Mar 09, 2021 at 08:14:50PM -0800, Andrii Nakryiko escreveu:
> On Tue, Mar 9, 2021 at 1:57 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> > On Tue, 2021-03-09 at 13:37 -0800, Andrii Nakryiko wrote:
> > > On Tue, Mar 9, 2021 at 3:48 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> > > > Em Tue, Mar 09, 2021 at 12:59:13AM +0100, Ilya Leoshkevich escreveu:
> > > TBH, I think it's not exactly right to call out libbpf version
> > > here.  It's BTF "version" (if we had such a thing) that determines
> > > the set of supported BTF kinds. There could be other libraries
> > > that might want to parse BTF. So I don't know what this should be
> > > called, but libbpf_compat is probably a wrong name for it.

> > BTF version seems to exist: btf_header.version. Should we maybe bump
> > this?

> That seems excessive. If the kernel doesn't use FLOATs, then no one
> would even notice a difference. While if we bump this version, then
> everything will automatically become incompatible.

> > > If we do want to teach pahole to not emit some parts of BTF, it
> > > should
> > > probably be a set of BPF features, not some arbitrary library
> > > versions.

> > I thought about just adding --btf-allow-floats, but if new features
> > will be added in the future, the list of options will become unwieldy.
> > So I thought it would be good to settle for something that increases
> > monotonically.

> BTF_KIND_FLOAT is the first extension in a long while. I'd worry about
> the proliferation of new options when we actually see some proof of
> that being a problem in practice.

I tend to agree, Ilya, can you rework the patch in that direction?
Something like --encode-btf-kind-float that starts disabled or other
suitable name?

- Arnaldo