[tip: x86/microcode] x86/cpu: Add platform ID to CPU matching structure

[tip-bot2 for Dave Hansen]

The following commit has been merged into the x86/microcode branch of tip:

Commit-ID:     c44b68639703434c52e4838f5080f0a02a2acaa6
Gitweb:        https://git.kernel.org/tip/c44b68639703434c52e4838f5080f0a02a2acaa6
Author:        Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate:    Wed, 04 Mar 2026 10:10:22 -08:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Thu, 05 Mar 2026 10:41:30 -08:00

x86/cpu: Add platform ID to CPU matching structure

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@xxxxxxxxxxxxxxx>
Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
Link: https://patch.msgid.link/20260304181022.058DF07C@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/match.c     | 3 +++
 include/linux/mod_devicetable.h | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/arch/x86/kernel/cpu/match.c b/arch/x86/kernel/cpu/match.c
index 6af1e8b..4604802 100644
--- a/arch/x86/kernel/cpu/match.c
+++ b/arch/x86/kernel/cpu/match.c
@@ -76,6 +76,9 @@ const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id *match)
 		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 --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
index 5b1725f..23ff240 100644
--- a/include/linux/mod_devicetable.h
+++ b/include/linux/mod_devicetable.h
@@ -691,6 +691,7 @@ 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;
 };
@@ -702,6 +703,7 @@ 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