Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Yonghong Song <yhs@fb.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	bpf <bpf@vger.kernel.org>,
	kernel-team@fb.com, Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Bill Wendling <morbo@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto
Date: Wed, 7 Apr 2021 10:13:24 -0300	[thread overview]
Message-ID: <YG2v9EY0WWp+bijr@kernel.org> (raw)
In-Reply-To: <CAKwvOdmX8d3XTzJFk5rN_PnOQYJ8bXMrh8DrhzqN=UBNdQiO3g@mail.gmail.com>

Em Fri, Apr 02, 2021 at 11:07:10AM -0700, Nick Desaulniers escreveu:
> On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@fb.com> wrote:
> > Currently, clang LTO built vmlinux won't work with pahole.
> > LTO introduced cross-cu dwarf tag references and broke
> > current pahole model which handles one cu as a time.
> > The solution is to merge all cu's as one pahole cu as in [1].
> > We would like to do this merging only if cross-cu dwarf
> > references happens. The LTO build mode is a pretty good
> > indication for that.

> > In earlier version of this patch ([2]), clang flag
> > -grecord-gcc-switches is proposed to add to compilation flags
> > so pahole could detect "-flto" and then merging cu's.
> > This will increate the binary size of 1% without LTO though.

> > Arnaldo suggested to use a note to indicate the vmlinux
> > is built with LTO. Such a cheap way to get whether the vmlinux
> > is built with LTO or not helps pahole but is also useful
> > for tracing as LTO may inline/delete/demote global functions,
> > promote static functions, etc.

> > So this patch added an elfnote with a new type LINUX_ELFNOTE_LTO_INFO.
> > The owner of the note is "Linux".

> > With gcc 8.4.1 and clang trunk, without LTO, I got
> >   $ readelf -n vmlinux
> >   Displaying notes found in: .notes
> >     Owner                Data size        Description
> >   ...
> >     Linux                0x00000004       func
> >      description data: 00 00 00 00
> >   ...
> > With "readelf -x ".notes" vmlinux", I can verify the above "func"
> > with type code 0x101.
> >
> > With clang thin-LTO, I got the same as above except the following:
> >      description data: 01 00 00 00
> > which indicates the vmlinux is built with LTO.
> >
> >   [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/
> >   [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/
> >
> > Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > Signed-off-by: Yonghong Song <yhs@fb.com>
> 
> LGTM thanks Yonghong!
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Thanks!

- Arnaldo