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

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Date:   Mon, 15 Jun 2020 14:47:27 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
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 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?

Sami