linux-kernel - Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA

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Message-ID: <CAK8P3a0cyAYqdPg_OoaVVY2Gjn=vtwAyfsVPjZ5qYddiOOc-eA@mail.gmail.com>
Date:   Fri, 26 Feb 2021 10:51:40 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Scull <ascull@...gle.com>,
        David Brazdil <dbrazdil@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

On Fri, Feb 26, 2021 at 10:05 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> On Fri, Feb 26, 2021 at 9:14 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> > > BTW, is CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y setable for x86 (64bit)?
> > > ( Did not look or check for it. )
> >
> > No, in mainline, HAVE_LD_DEAD_CODE_DATA_ELIMINATION is currently
> > only selected on MIPS and PowerPC. I only sent experimental patches to
> > enable it on arm64 and m68k, but have not tried booting them. If you
> > select the symbol on x86, you should see similar results.
> >
>
> OK, i see:
>
> $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/
> arch/mips/Kconfig:      select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>
> $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/
> arch/powerpc/Kconfig:   select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>
> So, I need to add this to arch/x86/Kconfig.
>
> You happen to know if changes to arch/x86/kernel/vmlinux.lds.S
> (sections) are needed?

No idea. I'm still debugging a possible regression on arm64, but both
issues I found for arm64 are specific to that architecture and won't
happen on x86. It's likely that something else breaks though.

> Last question:
> The last days I see a lot of fixes touching inlining with LLVM/Clang v13-git.
> What git tag are you using?
> What are your experiences?
> Pending patches (kernel-side)?
>
> I use:
> $ /opt/llvm-toolchain/bin/clang --version
> dileks clang version 13.0.0 (https://github.com/llvm/llvm-project.git
> c465429f286f50e52a8d2b3b39f38344f3381cce)

This is what I have on the build box:
Ubuntu clang version
13.0.0-++20210223104451+ebca13c66504-1~exp1~20210223095200.234

        Arnd