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

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Message-ID: <20170413231439.GG28657@google.com>
Date:   Thu, 13 Apr 2017 16:14:39 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     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>, 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

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.

Cheers

Matthias