Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o

From: Jiri Slaby <jslaby@suse.cz>
To: Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Masahiro Yamada <masahiroy@kernel.org>
Cc: Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>,
	Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o
Date: Tue, 21 Jun 2022 09:30:11 +0200	[thread overview]
Message-ID: <14c9839b-c015-69c1-84f0-a99d03877005@suse.cz> (raw)
In-Reply-To: <CAKwvOdmb5xdF70TzNp=4STCpzkGh16FnuKE1KbdzDhHt=OuRFA@mail.gmail.com>

On 17. 06. 22, 18:32, Nick Desaulniers wrote:
> On Fri, Jun 17, 2022 at 3:35 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>>
>> On Fri, Jun 17, 2022 at 12:53 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>>>
>>> On Thu, Jun 16, 2022 at 4:09 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>>>>
>>>> On Thu, Jun 16, 2022 at 12:45 PM Jiri Slaby <jslaby@suse.cz> wrote:
>>>>>
>>>>> Until the link-vmlinux.sh split (cf. the commit below), the linker was
>>>>> run with jobserver set in MAKEFLAGS. After the split, the command in
>>>>> Makefile.vmlinux_o is not prefixed by "+" anymore, so this information
>>>>> is lost.
>>>>>
>>>>> Restore it as linkers working in parallel (esp. the LTO ones) make a use
>>>>> of it.
> 
> TBH, I agree with Masahiro. I didn't understand this comment "esp. the
> LTO ones."  Jiri, can you clarify what you mean here?

Sure. clang is not the only compiler/linker that can handle kernel ;). 
So while rebasing gcc-LTO-kernel-patches to 5.19-rc (to upstream them 
later), this broke. See:
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/lto-wrapper.cc;h=26e06e77be4e0afb2bc3e913062a9c51cab5d205;hb=HEAD#l1336

gcc really parses MAKEFLAGS and looks for "--jobserver-auth=" there, if 
one passes specifies -flto=jobserver.

thanks,
-- 
js
suse labs