linux-kernel - Re: [PATCH] kbuild: pass jobserver to cmd_ld

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Date:   Tue, 20 Sep 2022 10:43:43 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH] kbuild: pass jobserver to cmd_ld_vmlinux.o

On 21. 06. 22, 10:41, Masahiro Yamada wrote:
> On Tue, Jun 21, 2022 at 4:30 PM Jiri Slaby <jslaby@...e.cz> wrote:
>>
>> On 17. 06. 22, 18:32, Nick Desaulniers wrote:
>>> On Fri, Jun 17, 2022 at 3:35 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>>>>
>>>> On Fri, Jun 17, 2022 at 12:53 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>>>>>
>>>>> On Thu, Jun 16, 2022 at 4:09 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>>>>>>
>>>>>> On Thu, Jun 16, 2022 at 12:45 PM Jiri Slaby <jslaby@...e.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.

Hi,

OK, I will make it part of the gcc-lto series.

> 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.

Right. First, gcc compiles the sources to an intermediate language. Then 
gcc-ld (LTO-)links the objects into a binary object. BFD ld/gold is 
invoked only from within gcc to actually link the binary.

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

With gcc-lto patchset, gcc-ld is invoked in cmd_ld_vmlinux.o.

So you are right, making it part of the gcc-lto patchset makes perfect 
sense. First, to see the context. Second, upstream doesn't need the change.

thanks,
-- 
js
suse labs