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

From: Masahiro Yamada <masahiroy@kernel.org>
To: Yonghong Song <yhs@fb.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Sedat Dilek <sedat.dilek@gmail.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 <kernel-team@fb.com>,
	Bill Wendling <morbo@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto
Date: Sun, 11 Apr 2021 21:31:12 +0900	[thread overview]
Message-ID: <CAK7LNASUkLi4gu-9TY7p7kaLFKtEFA1qA0kc3VtOcgH9xJgsfA@mail.gmail.com> (raw)
In-Reply-To: <08f2eda5-2226-d551-d660-dba981b6ced8@fb.com>

On Wed, Apr 7, 2021 at 11:49 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 4/7/21 6:46 AM, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Apr 06, 2021 at 11:23:27PM -0700, Yonghong Song escreveu:
> >> On 4/6/21 8:01 PM, Sedat Dilek wrote:
> >>> On Tue, Apr 6, 2021 at 6:13 PM Yonghong Song <yhs@fb.com> wrote:
> >>>> Masahiro and Michal,
> >
> >>>> Friendly ping. Any comments on this patch?
> >
> >>>> The addition LTO .notes information emitted by kernel is used by pahole
> >>>> in the following patch:
> >>>>       https://lore.kernel.org/bpf/20210401025825.2254746-1-yhs@fb.com/
> >>>>       (dwarf_loader: check .notes section for lto build info)
> >
> >>> the above pahole patch has this define and comment:
> >
> >>> -static bool cus__merging_cu(Dwarf *dw)
> >>> +/* Match the define in linux:include/linux/elfnote.h */
> >>> +#define LINUX_ELFNOTE_BUILD_LTO 0x101
> >
> >>> ...and does not fit with the define and comment in this kernel patch:
> >
> >>> +#include <linux/elfnote.h>
> >>> +
> >>> +#define LINUX_ELFNOTE_LTO_INFO 0x101
> >
> >> Thanks, Sedat. I am aware of this. I think we can wait in pahole
> >> to make a change until the kernel patch is finalized and merged.
> >> The kernel patch may still change as we haven't get
> >> maintainer's comment. This will avoid unnecessary churn's
> >> in pahole side.
> >
> > So, I tested with clang 12 on fedora rawhide as well on fedora 33, and
> > I'm satisfied with the current state to release v1.21, Masahiro, have
> > you had the time to look at this?
> >
> > Yonghong, as we have a fallback in case the ELF note isn't available, I
> > think we're safe even if the notes patch merge gets delayed, right?
>
> Right. That is why I separated the notes patch from other patches.
> We can revisit it once the kernel patch is settled.
>
> >
> > - Arnaldo
> >

Applied to linux-kbuild. Thanks.

-- 
Best Regards
Masahiro Yamada