linux-kernel - Re: [PATCH v1 00/26] x86: Introduce centralized CPUID model

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Message-ID: <aBnSgu_JyEi8fvog@gmail.com>
Date: Tue, 6 May 2025 11:12:34 +0200
From: Ingo Molnar <mingo@...nel.org>
To: "Ahmed S. Darwish" <darwi@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	John Ogness <john.ogness@...utronix.de>, x86@...nel.org,
	x86-cpuid@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 00/26] x86: Introduce centralized CPUID model


* Ahmed S. Darwish <darwi@...utronix.de> wrote:

>  MAINTAINERS                               |    1 +
>  arch/x86/include/asm/cpu.h                |    6 +
>  arch/x86/include/asm/cpuid.h              |    1 +
>  arch/x86/include/asm/cpuid/internal_api.h |   62 +
>  arch/x86/include/asm/cpuid/leaf_0x2_api.h |   57 +-
>  arch/x86/include/asm/cpuid/leaves.h       | 2055 +++++++++++++++++++++
>  arch/x86/include/asm/cpuid/table_api.h    |  120 ++
>  arch/x86/include/asm/cpuid/types.h        |   74 +
>  arch/x86/include/asm/processor.h          |    1 +
>  arch/x86/kernel/cpu/Makefile              |    2 +
>  arch/x86/kernel/cpu/cacheinfo.c           |  280 +--
>  arch/x86/kernel/cpu/common.c              |   65 +-
>  arch/x86/kernel/cpu/cpuid_debugfs.c       |   98 +
>  arch/x86/kernel/cpu/cpuid_scanner.c       |  209 +++
>  arch/x86/kernel/cpu/cpuid_scanner.h       |  117 ++
>  arch/x86/kernel/cpu/intel.c               |   17 +-
>  arch/x86/lib/cpu.c                        |   41 +-
>  tools/arch/x86/kcpuid/cpuid.csv           |    4 +-
>  18 files changed, 2926 insertions(+), 284 deletions(-)
>  create mode 100644 arch/x86/include/asm/cpuid/internal_api.h
>  create mode 100644 arch/x86/include/asm/cpuid/leaves.h
>  create mode 100644 arch/x86/include/asm/cpuid/table_api.h
>  create mode 100644 arch/x86/kernel/cpu/cpuid_debugfs.c
>  create mode 100644 arch/x86/kernel/cpu/cpuid_scanner.c
>  create mode 100644 arch/x86/kernel/cpu/cpuid_scanner.h

Regarding CPUID header organization:

 - Please move <asm/cpuid/internal_api.h> into <asm/cpuid/table_api.h>. 

   There's not really much point to making it 'internal' AFAICS, as the 
   main <asm/cpuid.h> header already includes <asm/cpuid/table_api.h> 
   and <asm/cpuid/internal_api.h> respectively, so it's not all so much 
   internal anymore.

 - Please just use a single central API header: <asm/cpuid/api.h>, and 
   remove <asm/cpuid.h>. It's confusing to have both <asm/cpuid.h> and 
   a proper <asm/cpuid/> header hierarchy.

   ( I wanted to point to <asm/fpu/api.h> as the shining example to 
     follow, but then I noticed that somehow we grew a <asm/fpu.h> wart 
     last year via b0b8a15bb89e. Will fix that ... )

 - Is there a strong reason to keep <asm/cpuid/leaf_0x2_api.h>? I think 
   for_each_leaf_0x2_entry() could just be moved into 
   <asm/cpuid/api.h>, it's one of the accessors.

 - In a similar vein, I don't see much point of keeping 
   <asm/cpuid/table_api.h> header separate either. <asm/cpuid/api.h> 
   won't be overly large I think.

 - Could we rename <asm/cpuid/leaves.h> to <asm/cpuid/leaf_types.h> or 
   so? It's really a sub-header of <asm/cpuid/types.h> and should thus 
   share the nomenclature.

 - After all this we'll only have 3 headers left:

	<asm/cpuid/types.h>
	<asm/cpuid/leaf_types.h>

	<asm/cpuid/api.h>

   And <asm/cpuid/leaf_types.h> is only a separate header because it's 
   autogenerated by an external project.

 - Wrt. <asm/cpuid/api.h>, we'll need a few followup cleanups there too 
   I think, such as migrating to the cpuid_*() namespace:

     - Rename have_cpuid_p() to cpuid_feature() or so.

     - I find the cpudata_cpuid_ namespace a bit confusing:

		__cpudata_cpuid_subleaf_idx(__table, __leaf, __subleaf, __idx)
		__cpudata_cpuid_subleaf(__table, __leaf, __subleaf)
		cpudata_cpuid_subleaf(_cpuinfo, _leaf, _subleaf)
		cpudata_cpuid(_cpuinfo, _leaf)
		cpudata_cpuid_nr_entries(_cpuinfo, _leaf)
		cpudata_cpuid_index(_cpuinfo, _leaf, _idx)
		cpudata_cpuid_regs(_cpuinfo, _leaf)
		cpudata_cpuid_index_regs(_cpuinfo, _leaf, _idx)

       All of CPUID processing is related to 'data', and we don't 
       really have any 'cpudata' primitives, so the cpudata_ prefix is 
       confusing to me.

       It's particularly confusing for methods like cpudata_cpuid(), 
       which sounds like a generic method, while in reality it accesses 
       subleaf 0, right? Why not name it cpuid_subleaf_0() or so?

       My suggestion would be to use a structure like this:

		__cpuid_subleaf_idx(__table, __leaf, __subleaf, __idx)
		__cpuid_subleaf(__table, __leaf, __subleaf)
		cpuid_subleaf(_cpuinfo, _leaf, _subleaf)
		cpuid_subleaf_0(_cpuinfo, _leaf)
		cpuid_leaf_nr_entries(_cpuinfo, _leaf)
		cpuid_leaf_index(_cpuinfo, _leaf, _idx)
		cpuid_leaf_regs(_cpuinfo, _leaf)
		cpuid_leaf_index_regs(_cpuinfo, _leaf, _idx)

        Or so? In my book it's a nice bonus that they thus become part 
        of the overall cpuid_*() API family. Note how these accessors 
        still are all still either cpuid_leaf_ or cpuid_subleaf_ 
        prefixed.

Thanks,

	Ingo