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
prev parent reply other threads:[~2021-04-07 13:13 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 23:27 [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto Yonghong Song
2021-04-02 18:07 ` Nick Desaulniers
2021-04-02 18:31 ` Sedat Dilek
2021-04-02 19:38 ` Sedat Dilek
2021-04-02 19:56 ` Sedat Dilek
2021-04-06 7:05 ` Sedat Dilek
2021-04-06 16:13 ` Yonghong Song
2021-04-07 3:01 ` Sedat Dilek
2021-04-07 6:23 ` Yonghong Song
2021-04-07 9:27 ` Sedat Dilek
2021-04-07 13:46 ` Arnaldo Carvalho de Melo
2021-04-07 14:49 ` Yonghong Song
2021-04-11 12:31 ` Masahiro Yamada
2021-04-11 17:43 ` Yonghong Song
2021-04-11 18:09 ` Sedat Dilek
2021-04-07 13:13 ` Arnaldo Carvalho de Melo [this message]
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