linux-kernel - [tip: x86/urgent] x86/build: Disable SSE4a

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Message-ID: <176168102963.2601451.953876733690125275.tip-bot2@tip-bot2>
Date: Tue, 28 Oct 2025 19:50:29 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 "Borislav Petkov (AMD)" <bp@...en8.de>,
 Arisu Tachibana <arisu.tachibana@...aclelinux.com>, christian.koenig@....com,
 Harry Wentland <harry.wentland@....com>,  <stable@...nel.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/build: Disable SSE4a

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     0d6e9ec80cebf9b378a1d3a01144e576d731c397
Gitweb:        https://git.kernel.org/tip/0d6e9ec80cebf9b378a1d3a01144e576d731c397
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Mon, 27 Oct 2025 12:40:59 +01:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Tue, 28 Oct 2025 20:43:36 +01:00

x86/build: Disable SSE4a

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>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Acked-by: Borislav Petkov (AMD) <bp@...en8.de>
Acked-by: Arisu Tachibana <arisu.tachibana@...aclelinux.com>
Acked-by: Christian König <christian.koenig@....com>
Acked-by: Harry Wentland <harry.wentland@....com>
Cc: <stable@...nel.org>
---
 arch/x86/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 4db7e4b..8fbff31 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