linux-kernel - Re: [PATCH 2/2] selftests/fpu: Add an FPU selftest

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Date:   Tue, 23 Jun 2020 20:47:26 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     X86 ML <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] selftests/fpu: Add an FPU selftest

On Tue, Jun 23, 2020 at 11:22:53AM -0700, Andy Lutomirski wrote:
> See that same atrocious bug report.  It's the insane interaction
> between -mno-sse2 and -mpreferred-stack-boundary.  So you need to
> cc-option them both?  Or just stick with a compiler version check, I
> guess.

Yes, it was the interaction. This below seems to work. Note the "-msse"
in the first argument of cc-option which causes the compiler to error
out with

/dev/null:1:0: error: -mpreferred-stack-boundary=3 is not between 4 and 12

Adding Nick for the clang side.

@Nick: I'm simply going to add -msse2 with cc-option.

Anyway, lemme test this thing a bit more.

Thx.

---

# CFLAGS for compiling floating point code inside the kernel. x86/Makefile turns
# off the generation of FPU/SSE* instructions for kernel proper but FPU_FLAGS
# get appended last to CFLAGS and thus override those previous compiler options.
#
FPU_CFLAGS := -mhard-float -msse
FPU_CFLAGS += $(call cc-option,-msse2,)
ifdef CONFIG_CC_IS_GCC
# Stack alignment mismatch, proceed with caution.
# GCC < 7.1 cannot compile code using `double` and -mpreferred-stack-boundary=3
# (8B stack alignment).
# See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
FPU_CFLAGS += $(call cc-option,-msse -mpreferred-stack-boundary=3,-mpreferred-stack-boundary=4)
endif

obj-$(CONFIG_TEST_FPU) += test_fpu.o
CFLAGS_test_fpu.o += $(FPU_CFLAGS)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette