[PATCH v4 1/4] x86/microcode: Refactor platform ID enumeration into a helper

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Today, the only code that cares about the platform ID is the microcode
update code itself. To facilitate storing the platform ID in a more
generic place and using it outside of the microcode update itself, put
the enumeration into a helper function. Mirror
intel_get_microcode_revision()'s naming and location.

But, moving away from intel_collect_cpu_info() means that the model
and family information in CPUID is not readily available. Just call
CPUID again.

Note that the microcode header is a mask of supported platform IDs.
Only stick the ID part in the helper. Leave the 1<<id part in the
microcode handling.

Also note that the PII is weird. It does not really have a platform
ID because it doesn't even have the MSR. Just consider it to be
platform ID 0. Instead of saying >=PII, say <=PII. The PII is the
real oddball here being the only CPU with Linux microcode updates
but no platform ID. It's worth calling it out by name.

This does subtly change the sig->pf for the PII though from 0x0
to 0x1. Make up for that by ignoring sig->pf when the microcode
update platform mask is 0x0.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Jon Kohler <jon@xxxxxxxxxxx>

--

Changes from v3:
 * Handle the empty platform mask on the PII

---

 b/arch/x86/kernel/cpu/microcode/intel.c |   53 +++++++++++++++++++++++++-------
 1 file changed, 42 insertions(+), 11 deletions(-)

diff -puN arch/x86/kernel/cpu/microcode/intel.c~refactor-get-processor-flags arch/x86/kernel/cpu/microcode/intel.c
--- a/arch/x86/kernel/cpu/microcode/intel.c~refactor-get-processor-flags	2026-03-04 09:19:02.438748485 -0800
+++ b/arch/x86/kernel/cpu/microcode/intel.c	2026-03-04 09:19:02.441748599 -0800
@@ -120,19 +120,43 @@ static inline unsigned int exttable_size
 	return et->count * EXT_SIGNATURE_SIZE + EXT_HEADER_SIZE;
 }
 
+/*
+ * Use CPUID to generate a "vfm" value. Useful
+ * before 'cpuinfo_x86' structures are populated.
+ */
+static u32 intel_cpuid_vfm(void)
+{
+	u32 eax   = cpuid_eax(1);
+	u32 fam   = x86_family(eax);
+	u32 model = x86_model(eax);
+
+	return IFM(fam, model);
+}
+
+static u32 intel_get_platform_id(void)
+{
+	unsigned int val[2];
+
+	/*
+	 * This can be called early. Use CPUID directly instead of
+	 * relying on cpuinfo_x86 which may not be fully initialized.
+	 * The PII does not have MSR_IA32_PLATFORM_ID. Everything
+	 * before _it_ has no microcode (for Linux at least).
+	 */
+	if (intel_cpuid_vfm() <= INTEL_PENTIUM_II_KLAMATH)
+		return 0;
+
+	/* get processor flags from MSR 0x17 */
+	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
+
+	return (val[1] >> 18) & 7;
+}
+
 void intel_collect_cpu_info(struct cpu_signature *sig)
 {
 	sig->sig = cpuid_eax(1);
-	sig->pf = 0;
 	sig->rev = intel_get_microcode_revision();
-
-	if (IFM(x86_family(sig->sig), x86_model(sig->sig)) >= INTEL_PENTIUM_III_DESCHUTES) {
-		unsigned int val[2];
-
-		/* get processor flags from MSR 0x17 */
-		native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
-		sig->pf = 1 << ((val[1] >> 18) & 7);
-	}
+	sig->pf  = 1 << intel_get_platform_id();
 }
 EXPORT_SYMBOL_GPL(intel_collect_cpu_info);
 
@@ -142,8 +166,15 @@ static inline bool cpu_signatures_match(
 	if (s1->sig != sig2)
 		return false;
 
-	/* Processor flags are either both 0 or they intersect. */
-	return ((!s1->pf && !pf2) || (s1->pf & pf2));
+	/*
+	 * Consider an empty mask to match everything. This
+	 * should only occur for one CPU model, the PII.
+	 */
+	if (!pf2)
+		return true;
+
+	/* Is the CPU's platform ID in the signature mask? */
+	return s1->pf & pf2;
 }
 
 bool intel_find_matching_signature(void *mc, struct cpu_signature *sig)
_