Re: [PATCH] arch/x86: Add an option to build the kernel with '-march=native' on x86-64

["H. Peter Anvin" <hpa@zytor.com>]

On March 21, 2025 7:28:58 AM PDT, Tor Vic <torvic9@mailbox.org> wrote:
>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 the CFLAGS.
>
>The idea comes from Linus' reply to Arnd's initial proposal in [1].
>
>This patch is based on Arnd's x86 cleanup series, which is now in -tip [2].
>
>[1] https://lore.kernel.org/all/CAHk-=wji1sV93yKbc==Z7OSSHBiDE=LAdG_d5Y-zPBrnSs0k2A@mail.gmail.com/
>[2] https://lore.kernel.org/all/20250226213714.4040853-1-arnd@kernel.org/
>
>Signed-off-by: Tor Vic <torvic9@mailbox.org>
>---
>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.
>---
> arch/x86/Kconfig.cpu | 9 +++++++++
> arch/x86/Makefile    | 5 +++++
> 2 files changed, 14 insertions(+)
>
>diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
>index 8fcb8ccee44b..057d7c28b794 100644
>--- a/arch/x86/Kconfig.cpu
>+++ b/arch/x86/Kconfig.cpu
>@@ -245,6 +245,15 @@ config MATOM
> 
> endchoice
> 
>+config NATIVE_CPU
>+	bool "Build for 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.
>+
> config X86_GENERIC
> 	bool "Generic x86 support"
> 	depends on X86_32
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index 8120085b00a4..0075bace3ed9 100644
>--- a/arch/x86/Makefile
>+++ b/arch/x86/Makefile
>@@ -178,8 +178,13 @@ else
> 	# Use -mskip-rax-setup if supported.
> 	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
> 
>+ifdef CONFIG_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

You may want to consider this option to also select matching kernel options (which would require probing cpuid.)