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

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Message-ID: <0914ae35-8feb-41f0-a6c7-6e96c43585cf@intel.com>
Date: Tue, 10 Feb 2026 15:27:05 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>, <linux-kernel@...r.kernel.org>
CC: <zhao1.liu@...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 2/6/2026 3:14 PM, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> 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.

"changes" used twice.

> 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.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>

Reviewed-by: Sohil Mehta <sohil.mehta@...el.com>