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

From: Yonghong Song <yhs@fb.com>
To: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>
Cc: <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@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: Wed, 7 Apr 2021 07:49:09 -0700	[thread overview]
Message-ID: <08f2eda5-2226-d551-d660-dba981b6ced8@fb.com> (raw)
In-Reply-To: <YG23xiRqJLYRtZgQ@kernel.org>

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
> 