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Sun, 25 Oct 2026 10:45:52 -0700 (PDT) Received: from antekk-kvm.a.lan ([45.137.227.125]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c46bd666561sm224235266b.54.2026.10.25.10.45.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 25 Oct 2026 10:45:51 -0700 (PDT) From: =?UTF-8?q?Antoni=20Kuczy=C5=84ski?= To: antoni.kucz@gmail.com, x86@kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] x86/build/64: Prevent native builds from generating SSE/AVX code Date: Sun, 25 Oct 2026 18:43:35 +0100 Message-ID: <20261025174336.8857-1-antoni.kucz@gmail.com> X-Mailer: git-send-email 2.47.3 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit With CONFIG_X86_NATIVE_CPU=y, GCC is invoked with -march=native after the -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a flags that keep FP/SIMD code out of the kernel. The GCC driver expands -march=native into an explicit list of ISA options for the build host (e.g. "-march=ivybridge ... -msse4.2 -mavx ..."), and because these come later on the command line they override the earlier -mno-* flags. As a result, on any build host with AVX the compiler is free to use SSE/AVX registers throughout the kernel, for instance to zero or copy structures. A trivial reproducer: struct s { long a, b, c, d; }; void f(struct s *p) { struct s z = {0}; *p = z; } $ gcc -mno-sse -mno-sse2 -mno-avx -march=native -Os -c t.c vpxor %xmm0,%xmm0,%xmm0 vmovdqu %ymm0,(%rdi) On an Ivy Bridge machine (i5-3317U, GCC 14.2) a v7.3 kernel built this way resets immediately after the bootloader hands over control. Booting the same image in QEMU with "-d int,cpu_reset -no-reboot" shows: v=06 (#UD) IP=0010:000000000125e47b v=0d (#GP) -> v=08 (#DF) -> Triple fault __pi_startup_64_load_idt+0xb: c5 f9 ef c0 vpxor %xmm0,%xmm0,%xmm0 The startup code executes an AVX instruction before CR4.OSXSAVE/XCR0 are set up and before an IDT is installed, so the #UD escalates to a triple fault. Even past early boot, compiler-generated SIMD code would silently clobber user space FPU state, since the kernel does not save it on kernel entry outside of kernel_fpu_begin()/kernel_fpu_end(). Re-apply the -mno-* flags after -march=native, the same way EGPR use is already suppressed for native builds. Fixes: ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'") Cc: stable@vger.kernel.org Signed-off-by: Antoni Kuczyński --- Notes: Tested: v7.3 + this patch, CONFIG_X86_NATIVE_CPU=y, GCC 14.2.0 (Debian), boots on i5-3317U. arch/x86/boot/startup/gdt_idt.o contains no xmm/ymm instructions after the change (previously 2). arch/x86/Makefile | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index b7e08519b8c1..2c0575fd7bbc 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -161,6 +161,9 @@ else ifdef CONFIG_X86_NATIVE_CPU KBUILD_CFLAGS += -march=native + # -march=native expands to explicit -msse/-mavx/... flags that + # override the earlier -mno-* ones; disable FP/SIMD code again. + KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a # Prevent the compiler from generating EGPR use. The kernel is # not yet prepared for general in-kernel use. KBUILD_CFLAGS += $(call cc-option,-mno-apx-features=egpr) -- 2.47.3