Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752631AbdFMGiH (ORCPT ); Tue, 13 Jun 2017 02:38:07 -0400 Received: from mail-wr0-f193.google.com ([209.85.128.193]:33152 "EHLO mail-wr0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752296AbdFMGiF (ORCPT ); Tue, 13 Jun 2017 02:38:05 -0400 Date: Tue, 13 Jun 2017 08:38:00 +0200 From: Ingo Molnar To: Matthias Kaehlcke Cc: Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , "H . J . Lu" , David Woodhouse , Masahiro Yamada , Michal Marek , x86@kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Davidson , Greg Hackmann , Nick Desaulniers , Stephen Hines , Kees Cook , Arnd Bergmann , Bernhard.Rosenkranzer@linaro.org, Peter Foley , Behan Webster , Douglas Anderson Subject: Re: [PATCH 3/3] x86/build: Specify stack alignment for clang Message-ID: <20170613063800.l24mai3lipzzypin@gmail.com> References: <20170613005531.77656-1-mka@chromium.org> <20170613005531.77656-4-mka@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170613005531.77656-4-mka@chromium.org> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2538 Lines: 68 * Matthias Kaehlcke wrote: > For gcc stack alignment is configured with -mpreferred-stack-boundary=N, > clang has the option -mstack-alignment=N for that purpose. Use the same > alignment as for gcc. > > If the alignment is not specified clang assumes an alignment of > 16 bytes, as required by the standard ABI. However as mentioned in > d9b0cde91c60 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if > supported") the standard kernel entry on x86-64 leaves the stack > on an 8-byte boundary, as a consequence clang will keep the stack > misaligned. > > Signed-off-by: Matthias Kaehlcke > --- > arch/x86/Makefile | 18 +++++++++++++----- > 1 file changed, 13 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile > index 86b725d69423..7f6c33f4d428 100644 > --- a/arch/x86/Makefile > +++ b/arch/x86/Makefile > @@ -11,6 +11,14 @@ else > KBUILD_DEFCONFIG := $(ARCH)_defconfig > endif > > +# Handle different option names for specifying stack alignment with gcc and > +# clang. > +ifeq ($(cc-name),clang) > + stack_align_opt := -mstack-alignment > +else > + stack_align_opt := -mpreferred-stack-boundary > +endif Nit: I'd name it cc_stack_align_opt or so, to make it clear this is a C compiler option. > @@ -65,8 +73,8 @@ ifeq ($(CONFIG_X86_32),y) > # with nonstandard options > KBUILD_CFLAGS += -fno-pic > > - # prevent gcc from keeping the stack 16 byte aligned > - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2) > + # prevent the compiler from keeping the stack 16 byte aligned > + KBUILD_CFLAGS += $(call cc-option,$(stack_align_opt)=2) So the comment appears inaccurate: the point isn't really to 'keep' the compiler from 16-byte alignment (there's nothing wrong with that, functionally), the point is to use a more optimal alignment, right? > > # Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use > # a lot more stack due to the lack of sharing of stacklots: > @@ -98,8 +106,8 @@ else > KBUILD_CFLAGS += $(call cc-option,-mno-80387) > KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387) > > - # Use -mpreferred-stack-boundary=3 if supported. > - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3) > + # Align the stack to 8 bytes if supported. > + KBUILD_CFLAGS += $(call cc-option,$(stack_align_opt)=3) Here too the reason should be outlined: performance, features or correctness? Thanks, Ingo