[tip: x86/cpu] x86/build: Rework CONFIG_GENERIC_CPU compiler flags

[tip-bot2 for Arnd Bergmann]

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

Commit-ID:     fc2d5cbe541032e74a66599ba843803cebbfed0e
Gitweb:        https://git.kernel.org/tip/fc2d5cbe541032e74a66599ba843803cebbfed0e
Author:        Arnd Bergmann <arnd@xxxxxxxx>
AuthorDate:    Wed, 26 Feb 2025 22:37:07 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 27 Feb 2025 11:19:05 +01:00

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 <arnd@xxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250226213714.4040853-4-arnd@xxxxxxxxxx
---
 arch/x86/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b773b3..5af3172 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