linux-kernel - Re: [PATCH 3/7] x86, LLVM: suppress clang warnings about unaligned accesses

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Date:   Thu, 13 Apr 2017 17:23:35 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Michael Davidson <md@...gle.com>, Michal Marek <mmarek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Shaohua Li <shli@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        x86@...nel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] x86, LLVM: suppress clang warnings about unaligned
 accesses

El Thu, Apr 13, 2017 at 04:55:00PM -0700 H. Peter Anvin ha dit:

> On 04/13/17 16:14, Matthias Kaehlcke wrote:
> > El Mon, Apr 03, 2017 at 04:01:58PM -0700 Matthias Kaehlcke ha dit:
> > 
> >> El Fri, Mar 17, 2017 at 04:50:19PM -0700 hpa@...or.com ha dit:
> >>
> >>> On March 16, 2017 5:15:16 PM PDT, Michael Davidson <md@...gle.com> wrote:
> >>>> Suppress clang warnings about potential unaliged accesses
> >>>> to members in packed structs. This gets rid of almost 10,000
> >>>> warnings about accesses to the ring 0 stack pointer in the TSS.
> >>>>
> >>>> Signed-off-by: Michael Davidson <md@...gle.com>
> >>>> ---
> >>>> arch/x86/Makefile | 5 +++++
> >>>> 1 file changed, 5 insertions(+)
> >>>>
> >>>> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> >>>> index 894a8d18bf97..7f21703c475d 100644
> >>>> --- a/arch/x86/Makefile
> >>>> +++ b/arch/x86/Makefile
> >>>> @@ -128,6 +128,11 @@ endif
> >>>>         KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args)
> >>>> endif
> >>>>
> >>>> +ifeq ($(cc-name),clang)
> >>>> +# Suppress clang warnings about potential unaligned accesses.
> >>>> +KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> >>>> +endif
> >>>> +
> >>>> ifdef CONFIG_X86_X32
> >>>> 	x32_ld_ok := $(call try-run,\
> >>>> 			/bin/echo -e '1: .quad 1b' | \
> >>>
> >>> Why conditional on clang?
> >>
> >> My understanding is that this warning is clang specific, it is not
> >> listed on https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
> > 
> > Actually this warning affects other platforms besides x86
> > (e.g. arm64), I'll submit a patch that disables the warning on all
> > platforms.
> > 
> 
> Drop the ifeq ($(cc-name),clang).
> 
> You should assume that if you have to add one of those ifeq's then you
> are doing something fundamentally wrong.

Thanks, however in the case of the global Makefile it seems we should
put it inside the already existing clang section:

http://lxr.free-electrons.com/source/Makefile#L692

Cheers

Matthias