x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'

Add a 'native' option that allows users to build an optimized kernel for
their local machine (i.e. the machine which is used to build the kernel)
by passing '-march=native' to CFLAGS.

The idea comes from Linus' reply to Arnd's initial proposal:

https://lore.kernel.org/all/CAHk-=wji1sV93yKbc==Z7OSSHBiDE=[e-mail]/

Here are some numbers comparing 'generic' to 'native' on a Skylake dual-core
laptop (generic --> native):

- vmlinux and compressed modules size:
125'907'744 bytes --> 125'595'280 bytes (-0.248 %)
18'810 kilobytes --> 18'770 kilobytes (-0.213 %)

- phoronix, average of 3 runs:
ffmpeg:
130.99 --> 131.15 (+0.122 %)
nginx:
10'650 --> 10'725 (+0.704 %)
hackbench (lower is better):
102.27 --> 99.50 (-2.709 %)

- xz compression of firefox tarball (lower is better):
319.57 seconds --> 317.34 seconds (-0.698 %)

- stress-ng, bogoops, average of 3 15-second runs:
fork:
111'744 --> 115'509 (+3.397 %)
bsearch:
7'211 --> 7'436 (+3.120 %)
memfd:
3'591 --> 3'604 (+0.362 %)
mmapfork:
630 --> 629 (-0.159 %)
schedmix:
42'715 --> 43'251 (+1.255 %)
epoll:
2'443'767 --> 2'454'413 (+0.436 %)
vm:
1'442'256 --> 1'486'615 (+3.076 %)

- schbench (two message threads), 30-second runs:
304 rps --> 305 rps (+0.329 %)

There is little difference both in terms of size and of performance, however
the native build comes out on top ever so slightly.

[ mingo: Renamed the option to CONFIG_X86_NATIVE_CPU, expanded the help text
and added Linus's Suggested-by tag. ]

Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Tor Vic <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/Kconfig.cpu

···
 
 endchoice
 
+config X86_NATIVE_CPU
+	bool "Build and optimize for local/native CPU"
+	depends on X86_64
+	default n
+	help
+	  Optimize for the current CPU used to compile the kernel.
+	  Use this option if you intend to build the kernel for your
+	  local machine.
+
+	  Note that such a kernel might not work optimally on a
+	  different x86 machine.
+
+	  If unsure, say N.
+
 config X86_GENERIC
 	bool "Generic x86 support"
 	depends on X86_32

arch/x86/Makefile

···
 	# Use -mskip-rax-setup if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
 
+ifdef CONFIG_X86_NATIVE_CPU
+        KBUILD_CFLAGS += -march=native
+        KBUILD_RUSTFLAGS += -Ctarget-cpu=native
+else
         KBUILD_CFLAGS += -march=x86-64 -mtune=generic
         KBUILD_RUSTFLAGS += -Ctarget-cpu=x86-64 -Ztune-cpu=generic
+endif
 
         KBUILD_CFLAGS += -mno-red-zone
         KBUILD_CFLAGS += -mcmodel=kernel