Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

From: Masahiro Yamada <masahiroy@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Nicholas Piggin <npiggin@gmail.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Kees Cook <keescook@chromium.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link
Date: Tue, 16 Jun 2020 12:22:20 +0900	[thread overview]
Message-ID: <CAK7LNARbZhoaA=Nnuw0=gBrkuKbr_4Ng_Ei57uafujZf7Xazgw@mail.gmail.com> (raw)
In-Reply-To: <CABCJKufaMU1z-s4S+dHhg0iCyVynLGwsFJgYnYyiBaR=mZt-8w@mail.gmail.com>

On Tue, Jun 16, 2020 at 6:47 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> On Sat, May 23, 2020 at 8:13 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > Hi Nicholas,
> > (+CC: Sam Ravnborg)
> >
> >
> > On Sat, May 23, 2020 at 7:06 PM Nicholas Piggin <npiggin@gmail.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