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

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Date:   Thu, 13 Apr 2017 16:55:00 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Matthias Kaehlcke <mka@...omium.org>
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>, x86@...nel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: [PATCH 3/7] x86, LLVM: suppress clang warnings about unaligned
 accesses

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.

	-hpa