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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 21 Jun 2022 17:41:18 +0900
Message-ID: <CAK7LNAS46hrVh3FNiHHL8_Hqp85VgWnsHyjQ4t-TFH_H344NOg@mail.gmail.com>
Subject: Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o
To:     Jiri Slaby <jslaby@suse.cz>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        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>
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On Tue, Jun 21, 2022 at 4:30 PM Jiri Slaby <jslaby@suse.cz> wrote:
>
> 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


Then, this patch should go together (or after) the GCC LTO upstreaming.


I still do not understand why you are patching the pre-modpost linking, though.

In my understanding, Clang LTO and GCC LTO are implemented
in different layers.

Clang LTO is implemented in the linker (i.e. lld).

In contrast,  GCC LTO is implemented in the compiler (i.e. gcc itself)
The linker (i.e. BFD linker or Gold from GNU binutils project)
is agnostic about LTO.


Your are patching the relocatable link ($(LD) -r),
but I could not find the code parsing jobserver-auth
in the binutils-gdb repository.


-- 
Best Regards
Masahiro Yamada