Re: [PATCH v4 2/4] x86/cpu: Add platform ID to CPU info structure

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Wed, Mar 04, 2026 at 10:10:20AM -0800, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> The end goal here is to be able to do x86_match_cpu() and match on a
> specific platform ID. While it would be possible to stash this ID
> off somewhere or read it dynamically, that approaches would not be
> consistent with the other fields which can be matched.
> 
> Read the platform ID and store it in cpuinfo_x86.
> 
> There are lots of sites to set this new field. Place it near
> the place c->microcode is established since the platform ID is
> so closely intertwined with microcode updates.
> 
> Note: This should not grow the size of 'struct cpuinfo_x86' in
> practice since the u8 fits next to another u8 in the structure.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
> Cc: Thomas Gleixner <tglx@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: x86@kernel.org
> Cc: Jon Kohler <jon@nutanix.com>
> 
> --
> 
> Changes from v2:
>  * rename x86_platform_id=>intel_platform_id and remove
>    initialization from generic code
>  * Add a amd_unused so it's clear the AMD folks have a
>    free field to play with.
> ---
> 
>  b/arch/x86/include/asm/microcode.h      |    2 ++
>  b/arch/x86/include/asm/processor.h      |    5 +++++
>  b/arch/x86/kernel/cpu/intel.c           |    1 +
>  b/arch/x86/kernel/cpu/microcode/intel.c |    2 +-
>  4 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff -puN arch/x86/include/asm/microcode.h~cpu-x86_stepping arch/x86/include/asm/microcode.h
> --- a/arch/x86/include/asm/microcode.h~cpu-x86_stepping	2026-02-24 15:37:21.947261472 -0800
> +++ b/arch/x86/include/asm/microcode.h	2026-02-24 15:37:21.955261858 -0800
> @@ -61,6 +61,8 @@ static inline int intel_microcode_get_da
>  	return hdr->datasize ? : DEFAULT_UCODE_DATASIZE;
>  }
>  
> +extern u32 intel_get_platform_id(void);
> +
>  static inline u32 intel_get_microcode_revision(void)
>  {
>  	u32 rev, dummy;
> diff -puN arch/x86/include/asm/processor.h~cpu-x86_stepping arch/x86/include/asm/processor.h
> --- a/arch/x86/include/asm/processor.h~cpu-x86_stepping	2026-02-24 15:37:21.949261568 -0800
> +++ b/arch/x86/include/asm/processor.h	2026-02-24 15:37:21.955261858 -0800
> @@ -140,6 +140,11 @@ struct cpuinfo_x86 {
>  		__u32		x86_vfm;
>  	};
>  	__u8			x86_stepping;
> +	union {
> +		// MSR_IA32_PLATFORM_ID[52-50]
> +		__u8			intel_platform_id;
> +		__u8			amd_unused;
> +	};
>  #ifdef CONFIG_X86_64
>  	/* Number of 4K pages in DTLB/ITLB combined(in pages): */
>  	int			x86_tlbsize;
> diff -puN arch/x86/kernel/cpu/intel.c~cpu-x86_stepping arch/x86/kernel/cpu/intel.c
> --- a/arch/x86/kernel/cpu/intel.c~cpu-x86_stepping	2026-02-24 15:37:21.950261617 -0800
> +++ b/arch/x86/kernel/cpu/intel.c	2026-02-24 15:37:21.955261858 -0800
> @@ -205,6 +205,7 @@ static void early_init_intel(struct cpui
>  
>  	if (c->x86 >= 6 && !cpu_has(c, X86_FEATURE_IA64))
>  		c->microcode = intel_get_microcode_revision();
> +	c->intel_platform_id = intel_get_platform_id();

Just want to make sure that c->intel_platform_id not getting explicitly
initialized for !CONFIG_CPU_SUP_INTEL is okay. I believe it is fine because
the global boot_cpu_data should be zero-initialized.

I see model and stepping are being initialized in identify_cpu(), but I
don't see any vendor specific initialization here:

identify_cpu()
{
        int i;

        c->loops_per_jiffy = loops_per_jiffy;
        c->x86_cache_size = 0;
        c->x86_vendor = X86_VENDOR_UNKNOWN;
        c->x86_model = c->x86_stepping = 0;     /* So far unknown... */

Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>