[PATCH v4 3/4] x86/cpu: Add platform ID to CPU matching structure

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.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 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.

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>
---

 b/arch/x86/kernel/cpu/match.c     |    3 +++
 b/include/linux/mod_devicetable.h |    2 ++
 2 files changed, 5 insertions(+)

diff -puN arch/x86/kernel/cpu/match.c~platform-match arch/x86/kernel/cpu/match.c
--- a/arch/x86/kernel/cpu/match.c~platform-match	2026-02-24 15:37:22.556290868 -0800
+++ b/arch/x86/kernel/cpu/match.c	2026-02-24 15:37:22.561291110 -0800
@@ -76,6 +76,9 @@ const struct x86_cpu_id *x86_match_cpu(c
 		if (m->steppings != X86_STEPPING_ANY &&
 		    !(BIT(c->x86_stepping) & m->steppings))
 			continue;
+		if (m->platform_mask != X86_PLATFORM_ANY &&
+		    !(BIT(c->intel_platform_id) & m->platform_mask))
+			continue;
 		if (m->feature != X86_FEATURE_ANY && !cpu_has(c, m->feature))
 			continue;
 		if (!x86_match_vendor_cpu_type(c, m))
diff -puN include/linux/mod_devicetable.h~platform-match include/linux/mod_devicetable.h
--- a/include/linux/mod_devicetable.h~platform-match	2026-02-24 15:37:22.558290965 -0800
+++ b/include/linux/mod_devicetable.h	2026-02-24 15:37:22.561291110 -0800
@@ -692,6 +692,7 @@ 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;
 };
@@ -703,6 +704,7 @@ struct x86_cpu_id {
 #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 */
 #define X86_CPU_TYPE_ANY 0
 
_