linux-kernel - Re: [PATCH 5/6] x86/cpu: Add platform ID to CPU matching structure

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Message-ID: <aW89OokcLlnFHlii@black.igk.intel.com>
Date: Tue, 20 Jan 2026 09:30:50 +0100
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, sohil.mehta@...el.com,
	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>, Jon Kohler <jon@...anix.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...nel.org>, Tony Luck <tony.luck@...el.com>,
	x86@...nel.org
Subject: Re: [PATCH 5/6] x86/cpu: Add platform ID to CPU matching structure

On Mon, Jan 19, 2026 at 11:50:57AM -0800, Dave Hansen wrote:

> The existing x86_match_cpu() infrastructure can be used to match
> a bunch of attributes of a CPU: vendor, family, model, steppings
> and CPU features.
> 
> But, there's one more attribute that's missing and unable to be
> matched against: the platform ID, enumerated on Intel CPUs in
> MSR_IA32_PLATFORM_ID. It is a little more obscure and is only
> queried during microcode loading. This is because Intel sometimes
> has CPUs with identical family/model/stepping but which need
> different microcode. These CPUs are differentiated with the
> platform ID.
> 
> Add a field in 'struct x86_cpu_id' for the platform ID. Similar
> to the stepping field, make the new field a mask of platform IDs.
> Some examples:
> 
> 	0x01: matches only platform ID 0x0
> 	0x02: matches only platform ID 0x1
> 	0x03: matches platform IDs 0x0 or 0x1
> 	0x80: matches only platform ID 0x7
> 	0xff: matches all 8 possible platform IDs
> 
> Since the mask is only a byte wide, it nestles in next to another
> u8 and does not even increase the size of 'struct x86_cpu_id'.
> 
> Reserve the all 0's value as the wildcard (X86_PLATFORM_ANY). This
> avoids forcing changes changes to existing 'struct x86_cpu_id' users.
> They can just continue to fill the field with 0's and their matching
> will work exactly as before.
> 
> Note: If someone is ever looking for space in 'struct x86_cpu_id',
> this new field could probably get stuck over in ->driver_data
> for the one user that there is.

...

> struct x86_cpu_id {

>  	__u16 feature;	/* bit index */
>  	/* Solely for kernel-internal use: DO NOT EXPORT to userspace! */
>  	__u16 flags;
> +	__u8  platform_mask;
>  	__u8  type;
>  	kernel_ulong_t driver_data;
>  };

...

>  #define X86_STEPPING_ANY 0
>  #define X86_STEP_MIN 0
>  #define X86_STEP_MAX 0xf
> +#define X86_PLATFORM_ANY 0x0
>  #define X86_FEATURE_ANY 0	/* Same as FPU, you can't test for that */

Wouldn't it be slightly better to locate the new one here to follow the order
in x86_cpu_id above?

>  #define X86_CPU_TYPE_ANY 0

-- 
With Best Regards,
Andy Shevchenko