Re: [PATCH 09/11] x86: rework CONFIG_GENERIC_CPU compiler flags

[Tor Vic]

On 12/4/24 11:30, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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 or a more recent level, add three separate options for
> v1, v2 and v3 of the architecture as defined by gcc and clang
> and make them all turn on CONFIG_GENERIC_CPU. Based on this it
> should be possible to change runtime feature detection into
> build-time detection for things like cmpxchg16b, or possibly
> gate features that are only available on older architectures.

Hi Arnd,

Similar but not identical changes have been proposed in the past several 
times like e.g. in 1, 2 and likely even more often.

Your solution seems to be much cleaner, I like it.

That said, on my Skylake platform, there is no difference between 
-march=x86-64 and -march=x86-64-v3 in terms of kernel binary size or 
performance.
I think Boris also said that these settings make no real difference on 
code generation.

Other settings might make a small difference (numbers are from 2023):
  -generic:       85.089.784 bytes
  -core2:         85.139.932 bytes
  -march=skylake: 85.017.808 bytes

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[1] 
https://lore.kernel.org/all/4_u6ZNYPbaK36xkLt8ApRhiRTyWp_-NExHCH_tTFO_fanDglEmcbfowmiB505heI4md2AuR9hS-VSkf4s90sXb5--AnNTOwvPaTmcgzRYSY=@proton.me/

[2] 
https://lore.kernel.org/all/20230707105601.133221-1-dimitri.ledkov@xxxxxxxxxxxxx/