[PATCH v3 03/10] x86: rework CONFIG_GENERIC_CPU compiler flags

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

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@arndb.de>
---
 arch/x86/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b773b34768d..5af3172fd51c 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
-- 
2.39.5