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

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 04 December 2024 10:31
> 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.
> 
> Link: https://lists.llvm.org/pipermail/llvm-dev/2020-July/143289.html
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/x86/Kconfig.cpu | 39 ++++++++++++++++++++++++++++++++++-----
>  arch/x86/Makefile    |  6 ++++++
>  2 files changed, 40 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
> index 139db904e564..1461a739237b 100644
> --- a/arch/x86/Kconfig.cpu
> +++ b/arch/x86/Kconfig.cpu
> @@ -260,7 +260,7 @@ endchoice
>  choice
>  	prompt "x86-64 Processor family"
>  	depends on X86_64
> -	default GENERIC_CPU
> +	default X86_64_V2
>  	help
>  	  This is the processor type of your CPU. This information is
>  	  used for optimizing purposes. In order to compile a kernel
> @@ -314,15 +314,44 @@ config MSILVERMONT
>  	  early Atom CPUs based on the Bonnell microarchitecture,
>  	  such as Atom 230/330, D4xx/D5xx, D2xxx, N2xxx or Z2xxx.
> 
> -config GENERIC_CPU
> -	bool "Generic-x86-64"
> +config X86_64_V1
> +	bool "Generic x86-64"
>  	depends on X86_64
>  	help
> -	  Generic x86-64 CPU.
> -	  Run equally well on all x86-64 CPUs.
> +	  Generic x86-64-v1 CPU.
> +	  Run equally well on all x86-64 CPUs, including early Pentium-4
> +	  variants lacking the sahf and cmpxchg16b instructions as well
> +	  as the AMD K8 and Intel Core 2 lacking popcnt.

The 'equally well' text was clearly always wrong (equally badly?)
but is now just 'plain wrong'.
Perhaps:
	Runs on all x86-64 CPUs including early cpu that lack the sahf,
	cmpxchg16b and popcnt instructions.

Then for V2 (or whatever it gets called)
	Requires support for the sahf, cmpxchg16b and popcnt instructions.
	This will not run on AMD K8 or Intel before Sandy bridge.

I think someone suggested that run-time detect of AVX/AVX2/AVX512
is fine?

	David

> +
> +config X86_64_V2
> +	bool "Generic x86-64 v2"
> +	depends on X86_64
> +	help
> +	  Generic x86-64-v2 CPU.
> +	  Run equally well on all x86-64 CPUs that meet the x86-64-v2
> +	  definition as well as those that only miss the optional
> +	  SSE3/SSSE3/SSE4.1 portions.
> +	  Examples of this include Intel Nehalem and Silvermont,
> +	  AMD Bulldozer (K10) and Jaguar as well as VIA Nano that
> +	  include popcnt, cmpxchg16b and sahf.
> +
> +config X86_64_V3
> +	bool "Generic x86-64 v3"
> +	depends on X86_64
> +	help
> +	  Generic x86-64-v3 CPU.
> +	  Run equally well on all x86-64 CPUs that meet the x86-64-v3
> +	  definition as well as those that only miss the optional
> +	  AVX/AVX2 portions.
> +	  Examples of this include the Intel Haswell and AMD Excavator
> +	  microarchitectures that include the bmi1/bmi2, lzncnt, movbe
> +	  and xsave instruction set extensions.
> 
>  endchoice
> 
> +config GENERIC_CPU
> +	def_bool X86_64_V1 || X86_64_V2 || X86_64_V3
> +
>  config X86_GENERIC
>  	bool "Generic x86 support"
>  	depends on X86_32
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 05887ae282f5..1fdc3fc6a54e 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -183,6 +183,9 @@ else
>          cflags-$(CONFIG_MPSC)		+= -march=nocona
>          cflags-$(CONFIG_MCORE2)		+= -march=core2
>          cflags-$(CONFIG_MSILVERMONT)	+= -march=silvermont
> +        cflags-$(CONFIG_MX86_64_V1)	+= -march=x86-64
> +        cflags-$(CONFIG_MX86_64_V2)	+= $(call cc-option,-march=x86-64-v2,-march=x86-64)
> +        cflags-$(CONFIG_MX86_64_V3)	+= $(call cc-option,-march=x86-64-v3,-march=x86-64)
>          cflags-$(CONFIG_GENERIC_CPU)	+= -mtune=generic
>          KBUILD_CFLAGS += $(cflags-y)
> 
> @@ -190,6 +193,9 @@ else
>          rustflags-$(CONFIG_MPSC)	+= -Ctarget-cpu=nocona
>          rustflags-$(CONFIG_MCORE2)	+= -Ctarget-cpu=core2
>          rustflags-$(CONFIG_MSILVERMONT)	+= -Ctarget-cpu=silvermont
> +        rustflags-$(CONFIG_MX86_64_V1)	+= -Ctarget-cpu=x86-64
> +        rustflags-$(CONFIG_MX86_64_V2)	+= -Ctarget-cpu=x86-64-v2
> +        rustflags-$(CONFIG_MX86_64_V3)	+= -Ctarget-cpu=x86-64-v3
>          rustflags-$(CONFIG_GENERIC_CPU)	+= -Ztune-cpu=generic
>          KBUILD_RUSTFLAGS += $(rustflags-y)
> 
> --
> 2.39.5
> 

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