linux-kernel - Re: [PATCH v2 2/2] kbuild: lto: preserve MAKEFLAGS for module linking

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Date:   Tue, 20 Sep 2022 14:34:47 +0200
From:   Martin Liška <mliska@...e.cz>
To:     Jiri Slaby <jirislaby@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 2/2] kbuild: lto: preserve MAKEFLAGS for module linking

On 9/20/22 10:06, Jiri Slaby wrote:
> On 19. 07. 22, 11:15, Masahiro Yamada wrote:
>> On Tue, Jul 19, 2022 at 5:41 PM Jiri Slaby <jslaby@...e.cz> wrote:
>>>
>>> From: Martin Liska <mliska@...e.cz>
>>>
>>> Prefix command in makefile run in order to preserve access to jobserver.
>>> This is needed for gcc at least.
>>>
>>> Fixes this warning:
>>> lto-wrapper: warning: jobserver is not available: ‘--jobserver-auth=’ is not present in ‘MAKEFLAGS’
>>
>>
>> What is lto-wrapper ?
> 
> lto-wrapper is a part of gcc:
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/lto-wrapper.cc
> 
> And is invoked during the link of LTO objects (also to link in parallel if jobserver is available).

Yes, it's a driver that is invoked by Linker when it decides to use LTO mode in GCC compiler.
Later on the lto-wrapper connects to make's jobserver and runs in parallel. Note a recent [1]
make implemented named pipes when it comes to communication with jobserver. And thus the '+'
won't be needed in the future, but we need it for now.

Cheers,
Martin

[1] https://git.savannah.gnu.org/cgit/make.git/commit/?id=7ad2593b2d2bb5b9332f4444d8bf93ac6f958bc6

> 
> I am leaving the unanswered questions below for Martin to answer.
> 
>>> Cc: Sedat Dilek <sedat.dilek@...il.com>
>>> Cc: Masahiro Yamada <masahiroy@...nel.org>
>>> Cc: Michal Marek <michal.lkml@...kovi.net>
>>> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
>>> Fixes: 5d45950dfbb1 (kbuild: move vmlinux.o link to scripts/Makefile.vmlinux_o)
>>> Signed-off-by: Martin Liska <mliska@...e.cz>
>>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>>> ---
>>>
>>> Notes:
>>>      [v2] this is new in v2
>>>
>>>   scripts/Makefile.build | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
>>> index bb63374fb055..493f3c4e8461 100644
>>> --- a/scripts/Makefile.build
>>> +++ b/scripts/Makefile.build
>>> @@ -247,8 +247,8 @@ endef
>>>
>>>   # Built-in and composite module parts
>>>   $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
>>> -       $(call if_changed_rule,cc_o_c)
>>> -       $(call cmd,force_checksrc)
>>> +       +$(call if_changed_rule,cc_o_c)
>>> +       +$(call cmd,force_checksrc)
>>
>>
>> cmd_force_checksrc invokes sparse.
>> How is it related to GCC?
>>
>>
>>
>>
>>>
>>>   # To make this rule robust against "Argument list too long" error,
>>>   # ensure to add $(obj)/ prefix by a shell command.
>>> @@ -457,7 +457,7 @@ endef
>>>   $(multi-obj-m): objtool-enabled := $(delay-objtool)
>>>   $(multi-obj-m): part-of-module := y
>>>   $(multi-obj-m): %.o: %.mod FORCE
>>> -       $(call if_changed_rule,ld_multi_m)
>>> +       +$(call if_changed_rule,ld_multi_m)
>>>   $(call multi_depend, $(multi-obj-m), .o, -objs -y -m)
>>
>>
>> cmd_ld_multi_m invokes $(LD) directly.
>> How is it related to gcc?
> 
> 
>