linux-kernel - Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

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Date:   Tue, 20 Jun 2017 10:37:21 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     hpa@...or.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>, x86@...nel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bernhard.Rosenkranzer@...aro.org,
        Peter Foley <pefoley2@...oley.com>,
        Behan Webster <behanw@...verseincode.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

El Tue, Jun 20, 2017 at 11:20:54AM +0200 Ingo Molnar ha dit:

> 
> * Matthias Kaehlcke <mka@...omium.org> wrote:
> 
> > Ingo didn't like the duplication and suggested the use of a variable, which 
> > kinda implies a check for the compiler name.
> 
> I don't think it implies that: why cannot cc_stack_align_opt probe for the 
> compiler option and use whichever is available, without hard-coding the compiler 
> name?

We could do this:

ifneq ($(call __cc-option, $(CC), -mno-sse, -mpreferred-stack-boundary=3,),)
        cc_stack_align_opt := -mpreferred-stack-boundary
endif
ifneq ($(call cc-option, -mstack-alignment=3,),)
        cc_stack_align_opt := -mstack-alignment
endif

If preferred cc-option could be used to probe for
-mpreferred-stack-boundary , however it would require REALMODE_CFLAGS
to be moved further down in the Makefile.

Since this solution also won't win a beauty price please let me know
if it is acceptable before respinning the patch or if you have other
suggestions.

> > I also think this is a cleaner solution. [...]
> 
> I concur with hpa: hard-coding compiler is awfully fragile and ugly as well.
> 
> With the proper probing of compiler options it will be possible for compilers to 
> consolidate their options, and it would be possible for a third compiler to use a 
> mixture of GCC and Clang options. With hard-coding none of that flexibility is 
> available.
> 
> > but I'm happy to respin the patch if you have another suggestion that is ok for 
> > both of you.
> 
> Please do.
> 
> Thanks,
> 
> 	Ingo