x86/cpu: Add platform ID to CPU info structure

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>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://patch.msgid.link/20260304181020.8D518228@davehans-spike.ostc.intel.com

arch/x86/include/asm/microcode.h

@@ -61,6 +61,8 @@ static inline int intel_microcode_get_datasize(struct microcode_header_intel *hd
 	return hdr->datasize ? : DEFAULT_UCODE_DATASIZE;
 }
 
+extern u32 intel_get_platform_id(void);
+
 static inline u32 intel_get_microcode_revision(void)
 {
 	u32 rev, dummy;

arch/x86/include/asm/processor.h

@@ -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;

arch/x86/kernel/cpu/intel.c

@@ -205,6 +205,7 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 
 	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) ||

arch/x86/kernel/cpu/microcode/intel.c

@@ -134,7 +134,7 @@ 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];