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

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Date:   Fri, 19 Jul 2019 10:17:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Vaibhav Rustagi <vaibhavrustagi@...gle.com>,
        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 Thu, Jul 18, 2019 at 02:34:44PM -0700, Nick Desaulniers wrote:
> 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.

Should we then not fix/remove these := assignments?