linux-kernel - Re: x86: Disable SSE4A

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Message-ID: <CANgtXuN-grBSdVfhtZ0xBk8pWyh6tUiMtAWOaPHBApQ3wrUy6w@mail.gmail.com>
Date: Tue, 28 Oct 2025 13:19:45 +0900
From: Arisu Tachibana <arisu.tachibana@...aclelinux.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>, x86@...nel.org, Leyvi Rose <leyvirose@...il.com>, 
	Samuel Holland <samuel.holland@...ive.com>, Christian König <christian.koenig@....com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: x86: Disable SSE4A

On Mon, Oct 27, 2025 at 9:41 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Oct 27, 2025 at 12:40:59PM +0100, Peter Zijlstra wrote:
> > Leyvi Rose reported that his X86_NATIVE_CPU=y build is failing because
> > our instruction decoder doesn't support SSE4A and the AMDGPU code seems
> > to be generating those with his compiler of choice (CLANG+LTO).
> >
> > Now, our normal build flags disable SSE MMX SSE2 3DNOW AVX, but then
> > CC_FLAGS_FPU re-enable SSE SSE2.
> >
> > Since nothing mentions SSE3 or SSE4, I'm assuming that -msse (or its
> > negative) control all SSE variants -- but why then explicitly enumerate
> > SSE2 ?
> >
> > Anyway, until the instruction decoder gets fixed, explicitly disallow
> > SSE4A (an AMD specific SSE4 extension).
> >
> > Fixes: ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 4db7e4bf69f5..8fbff3106c56 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -75,7 +75,7 @@ export BITS
> >  #
> >  #    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
> >  #
> > -KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
> > +KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a
> >  KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
> >  KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
>
> Acked-by: Borislav Petkov (AMD) <bp@...en8.de>
>

 Acked-by: Arisu Tachibana <arisu.tachibana@...aclelinux.com>


--
thanks,
arisut