linux-kernel - Re: [tip: x86/build] x86/build: Remove stale cc-option checks

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Message-ID: <CAKwvOd=ML1ytp8Q10oiz8q1ERAHcGnjjCSMOHj=tq6E2vHAkQw@mail.gmail.com>
Date:   Thu, 26 Aug 2021 11:55:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Nathan Chancellor <nathan@...nel.org>, x86@...nel.org,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [tip: x86/build] x86/build: Remove stale cc-option checks

On Sun, Aug 22, 2021 at 1:31 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * tip-bot2 for Nick Desaulniers <tip-bot2@...utronix.de> wrote:
>
> > The following commit has been merged into the x86/build branch of tip:
> >
> > Commit-ID:     1463c2a27d59c69358ad1cbd869d3a8649695d8c
> > Gitweb:        https://git.kernel.org/tip/1463c2a27d59c69358ad1cbd869d3a8649695d8c
> > Author:        Nick Desaulniers <ndesaulniers@...gle.com>
> > AuthorDate:    Thu, 12 Aug 2021 11:38:48 -07:00
> > Committer:     Borislav Petkov <bp@...e.de>
> > CommitterDate: Sun, 15 Aug 2021 10:32:52 +02:00
> >
> > x86/build: Remove stale cc-option checks
> >
> > -mpreferred-stack-boundary= is specific to GCC, while -mstack-alignment=
> > is specific to Clang. Rather than test for this three times via
> > cc-option and __cc-option, rely on CONFIG_CC_IS_* from Kconfig.
> >
> > GCC did not support values less than 4 for -mpreferred-stack-boundary=
> > until GCC 7+. Change the cc-option test to check for a value of 2,
> > rather than 4.
>
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -14,10 +14,13 @@ endif
> >
> >  # For gcc stack alignment is specified with -mpreferred-stack-boundary,
> >  # clang has the option -mstack-alignment for that purpose.
> > -ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)
> > +ifdef CONFIG_CC_IS_GCC
> > +ifneq ($(call cc-option, -mpreferred-stack-boundary=2),)
> >        cc_stack_align4 := -mpreferred-stack-boundary=2
> >        cc_stack_align8 := -mpreferred-stack-boundary=3
> > -else ifneq ($(call cc-option, -mstack-alignment=16),)
> > +endif
> > +endif
> > +ifdef CONFIG_CC_IS_CLANG
> >        cc_stack_align4 := -mstack-alignment=4
> >        cc_stack_align8 := -mstack-alignment=8
>
> So I spent most of yesterday bisecting a hard to diagnose bug that looked
> like a GPU driver bug - but the bisect somewhat surprisingly ended up at
> this commit.

I'm genuinely sorry about that.  Let me guess, GPF on SSE instruction
with stack based operand from AMDGPU? (I've seen that twice so far
related to these options.)

I see now what went wrong....
GCC only supports a 4B stack alignment for ***32b*** (or 16b) x86;
`-mpreferred-stack-boundary=2` will produce an error unless -m32 or
-m16 is set; but `-mpreferred-stack-boundary=2 -m32` has been long
supported.  It's -mpreferred-stack-boundary=3 for -m64 that wasn't
supported until the gcc-7 release.  So the cc-option test should
instead test -mpreferred-stack-boundary=3.

>
> Doing the partial revert below solves the regression - as the above hunk is
> not obviously an identity transformation. I have a pretty usual GCC 10.3.0
> build environment with nothing exotic.
>
> I amdended the commit with the partial revert in tip:x86/build.

No worries. I'll send a follow up.

-- 
Thanks,
~Nick Desaulniers