linux-kernel - Re: [PATCH 2/2] s390: boot, purgatory: pass $(CLANG

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Date:   Mon, 15 Apr 2019 08:32:34 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Philipp Rudo <prudo@...ux.ibm.com>,
        Hendrik Brueckner <brueckner@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] s390: boot, purgatory: pass $(CLANG_FLAGS) where
 needed

On Thu, 11 Apr 2019 11:08:31 -0700
Nick Desaulniers <ndesaulniers@...gle.com> wrote:

> On Thu, Apr 11, 2019 at 1:52 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Thu, Apr 11, 2019 at 12:14 AM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@...glegroups.com> wrote:  
> > > On Wed, Apr 10, 2019 at 1:13 PM Arnd Bergmann <arnd@...db.de> wrote:  
> > > >
> > > > The purgatory and boot Makefiles do not inherit the original cflags,
> > > > so clang falls back to the default target architecture when building it,
> > > > typically this would be x86 when cross-compiling.
> > > >
> > > > Add $(CLANG_FLAGS) everywhere so we pass the correct --target=s390x-linux
> > > > option when cross-compiling.
> > > >
> > > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > > > ---
> > > >  arch/s390/Makefile           | 5 +++--
> > > >  arch/s390/purgatory/Makefile | 1 +
> > > >  2 files changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/arch/s390/Makefile b/arch/s390/Makefile
> > > > index 9c079a506325..443990791099 100644
> > > > --- a/arch/s390/Makefile
> > > > +++ b/arch/s390/Makefile
> > > > @@ -17,12 +17,13 @@ KBUILD_CFLAGS_MODULE += -fPIC
> > > >  KBUILD_AFLAGS  += -m64
> > > >  KBUILD_CFLAGS  += -m64
> > > >  aflags_dwarf   := -Wa,-gdwarf-2
> > > > -KBUILD_AFLAGS_DECOMPRESSOR := -m64 -D__ASSEMBLY__
> > > > +KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
> > > >  KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
> > > > -KBUILD_CFLAGS_DECOMPRESSOR := -m64 -O2
> > > > +KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2
> > > >  KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
> > > >  KBUILD_CFLAGS_DECOMPRESSOR += -fno-delete-null-pointer-checks -msoft-float
> > > >  KBUILD_CFLAGS_DECOMPRESSOR += -fno-asynchronous-unwind-tables  
> > >
> > > Thanks for the respin with Nathan's suggestion.
> > >  
> > > > +KBUILD_CFLAGS_DECOMPRESSOR += $(call cc-disable-warning,pointer-sign)  
> > >
> > > What's up with this ^ ?  Seems like the top level sets it (without
> > > cc-disable-warning :( ), but then KBUILD_CFLAGS_DECOMPRESSOR discards
> > > it.  Does Clang actually flag code in this arch (that GCC doesn't)?  
> >
> > Oops, that should have been a separate patch.
> >
> > I think what happens is that clang warns more aggressively about pointer sign
> > bugs than gcc in some cases, and some of those cases happen in s390
> > header files that are included by both the kernel and the decompressor.
> >
> > The full warning log without this change is rather long, see
> > https://pastebin.com/KG9xaTNB  
> 
> From this link, it looks like the definitions of:
> __atomic64_or
> __atomic64_and
> __atomic64_xor
> and their *_barrier variants are problematic.  I think converting
> those to use unsigned long is the way to go.  Shouldn't you be doing
> bitwise ops on unsigned types anyways?

These functions follow the type of atomic64_t which is a "long" wrapped
in a structure. We do not want to change that to unsigned long, are we?
Then having some of the functions operate on "long" and others on
"unsigned long" seem odd.

> The warnings with __atomic64_add are tougher to read/understand since
> at that point the log lines look like they start to mix together.
> 
> >
> > I also tried patching the code to avoid the warnings, but I'm not entirely
> > happy with that result either, see
> > https://pastebin.com/pSMz5eZA  
> 
> That's no terrible, IMO, particularly with the change I suggest above.

That is not too bad, the only change I do not like is the s/u8/char/ in
struct ipl_block_fcp.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.