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

["Arnd Bergmann" <arnd@kernel.org>]

On Fri, Mar 21, 2025, at 15:28, Tor Vic 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].

Thanks for having another look at this and for including the
benchmarks. I ended up dropping this bit of my series because
there were too many open questions around things like
reproducible builds, but there is clearly a demand for having
this included.

>       hackbench (lower is better):
>       102.27 --> 99.50                         (-2.709 %)
>
>   - stress-ng, bogoops, average of 3 15-second runs:
>       fork:
>       111'744 --> 115'509                      (+3.397 %)
>       bsearch:
>       7'211 --> 7'436                          (+3.120 %)
>       vm:
>       1'442'256 --> 1'486'615                  (+3.076 %)

3% in userspace benchmarks does seem significant enough to
spend more time on seeing what exactly made the difference
here, and possibly including it as separate options.

> +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

I assume that the difference here is that -march=native on
your machine gets turned into -march=skylake, which then turns
on both additional instructions and a different instruction
scheduler.

Are you able to quickly run the same tests again using
just one of the two?

a) -march=x86-64 -mtune=skylake
b) -march=skylake -mtune=generic

    Arnd