linux-kernel - Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux

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Message-ID: <CAK7LNARbZhoaA=Nnuw0=gBrkuKbr_4Ng_Ei57uafujZf7Xazgw@mail.gmail.com>
Date:   Tue, 16 Jun 2020 12:22:20 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

On Tue, Jun 16, 2020 at 6:47 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> On Sat, May 23, 2020 at 8:13 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > Hi Nicholas,
> > (+CC: Sam Ravnborg)
> >
> >
> > On Sat, May 23, 2020 at 7:06 PM Nicholas Piggin <npiggin@...il.com> wrote:
> > >
> > > Excerpts from Masahiro Yamada's message of May 23, 2020 3:44 am:
> > > > + Michael, and PPC ML.
> > > >
> > > > They may know something about the reason of failure.
> > >
> > > Because the linker can't put branch stubs within object code sections,
> > > so when you incrementally link them too large, the linker can't resolve
> > > branches into other object files.
> >
> >
> > Ah, you are right.
> >
> > So, this is a problem not only for PPC
> > but also for ARM (both 32 and 64 bit), etc.
> >
> > ARM needs to insert a veneer to jump far.
> >
> > Prior to thin archive, we could not compile
> > ARCH=arm allyesconfig because
> > drivers/built-in.o was too large.
> >
> > This patch gets us back to the too large
> > incremental object situation.
> >
> > With my quick compile-testing,
> > ARCH=arm allyesconfig
> > and ARCH=arm64 allyesconfig are broken.
>
> Thanks for looking into this! Clang doesn't appear to have this issue
> with LTO because it always enables both -ffunction-sections and
> -fdata-sections. I confirmed that -ffunction-sections also fixes arm64
> allyesconfig with this patch. While I'm fine with reusing vmlinux.o
> only with LTO, how would you feel about enabling -ffunction-sections
> in the kernel by default?


I am OK if it works.

Please do compile tests for some architectures.
(especially, ARCH=powerpc defconfig, and ARCH=arm(64) allyesconfig)


Thank you.




-- 
Best Regards
Masahiro Yamada