linux-kernel - Re: [PATCH 1/2] x86/purgatory: add -mno-sse, -mno-mmx, -mno-sse2 to Makefile

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Message-ID: <CAKwvOdkHHNR7utufOcDwAOgBEA9MnOLh713Gaq01R=n26EyjZw@mail.gmail.com>
Date:   Thu, 18 Jul 2019 14:34:44 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Vaibhav Rustagi <vaibhavrustagi@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/purgatory: add -mno-sse, -mno-mmx, -mno-sse2 to Makefile

On Wed, Jul 17, 2019 at 5:02 PM Vaibhav Rustagi
<vaibhavrustagi@...gle.com> wrote:
>
> Compiling the purgatory code with clang results in using of mmx
> registers.
>
> $ objdump -d arch/x86/purgatory/purgatory.ro | grep xmm
>
>      112:       0f 28 00                movaps (%rax),%xmm0
>      115:       0f 11 07                movups %xmm0,(%rdi)
>      122:       0f 28 00                movaps (%rax),%xmm0
>      125:       0f 11 47 10             movups %xmm0,0x10(%rdi)
>
> Add -mno-sse, -mno-mmx, -mno-sse2 to avoid generating SSE instructions.
>
> Signed-off-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> ---
>  arch/x86/purgatory/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index 3cf302b26332..3589ec4a28c7 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -20,6 +20,7 @@ KCOV_INSTRUMENT := n
>  # sure how to relocate those. Like kexec-tools, use custom flags.
>
>  KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes -fno-zero-initialized-in-bss -fno-builtin -ffreestanding -c -Os -mcmodel=large
> +KBUILD_CFLAGS += -mno-mmx -mno-sse -mno-sse2

Yep, this is a commonly recurring bug in the kernel, observed again
and again for Clang builds.  The top level Makefile carefully sets
KBUILD_CFLAGS, then lower subdirs in the kernel wipe them away with
`:=` assignment. Invariably important flags don't always get re-added.
In this case, these flags are used in arch/x86/Makefile, but not here
and should be IMO.  Thanks for the patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

(Note that arch/x86/Makefile additionally sets -mno-3dnow and -mno-avx
(if supported by the compiler).  Not sure if the maintainers would
like a v2 with those added, and we don't strictly need them yet, but
we may someday).
-- 
Thanks,
~Nick Desaulniers