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	2026-02-13 15:51:01.027353195 -0800
+++ b/arch/x86/include/asm/microcode.h	2026-02-13 15:51:01.069354824 -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	2026-02-13 15:51:01.032353389 -0800
+++ b/arch/x86/include/asm/processor.h	2026-02-13 15:51:01.069354824 -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	2026-02-13 15:51:01.045353894 -0800
+++ b/arch/x86/kernel/cpu/intel.c	2026-02-13 15:51:01.069354824 -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();
 
 	/* Now if any of them are set, check the blacklist and clear the lot */
 	if ((cpu_has(c, X86_FEATURE_SPEC_CTRL) ||
diff -puN arch/x86/kernel/cpu/microcode/intel.c~cpu-x86_stepping arch/x86/kernel/cpu/microcode/intel.c
--- a/arch/x86/kernel/cpu/microcode/intel.c~cpu-x86_stepping	2026-02-13 15:51:01.066354708 -0800
+++ b/arch/x86/kernel/cpu/microcode/intel.c	2026-02-13 15:51:01.069354824 -0800
@@ -133,7 +133,7 @@ static u32 intel_cpuid_vfm(void)
 	return IFM(fam, model);
 }
 
-static u32 intel_get_platform_id(void)
+u32 intel_get_platform_id(void)
 {
 	unsigned int val[2];
 
_