linux-kernel - Re: [PATCH 1/2] s390: only build for new CPUs with clang

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Message-ID: <CAK8P3a175Cy5ojmh+EAEz9E8tjXKPkwpXsKg5qc7zxZd6OWUhQ@mail.gmail.com>
Date:   Thu, 11 Apr 2019 12:19:18 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Philipp Rudo <prudo@...ux.vnet.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] s390: only build for new CPUs with clang

On Thu, Apr 11, 2019 at 8:24 AM Heiko Carstens
<heiko.carstens@...ibm.com> wrote:
>
> On Wed, Apr 10, 2019 at 10:12:40PM +0200, Arnd Bergmann wrote:
> > llvm does does not understand -march=z9-109 and older target
> > specifiers, so disable the respective Kconfig settings and
> > the logic to make the boot code work on old systems when
> > building with clang.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  arch/s390/Kconfig       | 6 ++++++
> >  arch/s390/boot/Makefile | 2 ++
> >  2 files changed, 8 insertions(+)
> ...
> > diff --git a/arch/s390/boot/Makefile b/arch/s390/boot/Makefile
> > index c844eaf24ed7..953a74d04990 100644
> > --- a/arch/s390/boot/Makefile
> > +++ b/arch/s390/boot/Makefile
> > @@ -11,6 +11,7 @@ KASAN_SANITIZE := n
> >  KBUILD_AFLAGS := $(KBUILD_AFLAGS_DECOMPRESSOR)
> >  KBUILD_CFLAGS := $(KBUILD_CFLAGS_DECOMPRESSOR)
> >
> > +ifndef CONFIG_CC_IS_CLANG
> >  #
> >  # Use -march=z900 for als.c to be able to print an error
> >  # message if the kernel is started on a machine which is too old
> > @@ -25,6 +26,7 @@ CFLAGS_als.o                        += -march=z900
> >  CFLAGS_REMOVE_sclp_early_core.o      += $(CC_FLAGS_MARCH)
> >  CFLAGS_sclp_early_core.o     += -march=z900
> >  endif
> > +endif
>
> This contradicts the whole purpose of als.c - printing an error
> message to the console if the kernel is compiled for a newer
> architecture than it is running on (and therefore uses instructions
> unknown to the current system).
> If this can't be fixed/changed in clang, then it should be at least
> changed to the lowest possible architecture.

Ok, I'll do that. I originally did something like it, but then went back
to the simpler workaround to just make it compile.

      Arnd