From fc2d5cbe541032e74a66599ba843803cebbfed0e Mon Sep 17 00:00:00 2001 From: Arnd Bergmann Date: Wed, 26 Feb 2025 22:37:07 +0100 Subject: [PATCH] x86/build: Rework CONFIG_GENERIC_CPU compiler flags Building an x86-64 kernel with CONFIG_GENERIC_CPU is documented to run on all CPUs, but the Makefile does not actually pass an -march= argument, instead relying on the default that was used to configure the toolchain. In many cases, gcc will be configured to -march=x86-64 or -march=k8 for maximum compatibility, but in other cases a distribution default may be either raised to a more recent ISA, or set to -march=native to build for the CPU used for compilation. This still works in the case of building a custom kernel for the local machine. The point where it breaks down is building a kernel for another machine that is older the the default target. Changing the default to -march=x86-64 would make it work reliable, but possibly produce worse code on distros that intentionally default to a newer ISA. To allow reliably building a kernel for either the oldest x86-64 CPUs, pass the -march=x86-64 flag to the compiler. This was not possible in early versions of x86-64 gcc, but works on all currently supported versions down to at least gcc-5. Signed-off-by: Arnd Bergmann Signed-off-by: Ingo Molnar Cc: Linus Torvalds Link: https://lore.kernel.org/r/20250226213714.4040853-4-arnd@kernel.org --- arch/x86/Makefile | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 5b773b34768d1..5af3172fd51cd 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -183,14 +183,14 @@ else cflags-$(CONFIG_MPSC) += -march=nocona cflags-$(CONFIG_MCORE2) += -march=core2 cflags-$(CONFIG_MATOM) += -march=atom - cflags-$(CONFIG_GENERIC_CPU) += -mtune=generic + cflags-$(CONFIG_GENERIC_CPU) += -march=x86-64 -mtune=generic KBUILD_CFLAGS += $(cflags-y) rustflags-$(CONFIG_MK8) += -Ctarget-cpu=k8 rustflags-$(CONFIG_MPSC) += -Ctarget-cpu=nocona rustflags-$(CONFIG_MCORE2) += -Ctarget-cpu=core2 rustflags-$(CONFIG_MATOM) += -Ctarget-cpu=atom - rustflags-$(CONFIG_GENERIC_CPU) += -Ztune-cpu=generic + rustflags-$(CONFIG_GENERIC_CPU) += -Ctarget-cpu=x86-64 -Ztune-cpu=generic KBUILD_RUSTFLAGS += $(rustflags-y) KBUILD_CFLAGS += -mno-red-zone