From 74d68ead7218220505704d82820947746f70e282 Mon Sep 17 00:00:00 2001
From: "Chloe M." <chloe@mensia.org>
Date: Fri, 26 Jun 2026 20:46:27 +0000
Subject: [PATCH] stos/amd64: lapic: Detect if x2APIC mode is present

Signed-off-by: Chloe M. <chloe@mensia.org>
---
 paw/stos/arch/amd64/cpu/lapic.c | 14 ++++++++++++++
 paw/stos/head/arch/amd64/mcb.h  |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/paw/stos/arch/amd64/cpu/lapic.c b/paw/stos/arch/amd64/cpu/lapic.c
index 00662e1..8004f43 100644
--- a/paw/stos/arch/amd64/cpu/lapic.c
+++ b/paw/stos/arch/amd64/cpu/lapic.c
@@ -43,6 +43,19 @@ LapicIsSupported(VOID)
     return ISSET(Edx, BIT(9)) != 0;
 }
 
+/*
+ * Returns true if the CPU can operate its Local APIC unit
+ * in the newer x2APIC mode.
+ */
+static BOOLEAN
+LapicHasX2Apic(VOID)
+{
+    ULONG Ecx, Unused;
+
+    CPUID(1, Unused, Unused, Ecx, Unused);
+    return ISSET(Ecx, BIT(21)) != 0;
+}
+
 /*
  * Enable the Local APIC unit
  */
@@ -85,6 +98,7 @@ MdLapicInit(KPCR *Kpcr)
 
     Mcb = &Kpcr->Mcb;
     Mcb->LapicBase = PMA_TO_VMA((UPTR)Madt->LapicAddr);
+    Mcb->HasX2Apic = LapicHasX2Apic();
 
     /* Enable the Local APIC */
     LapicEnable();
diff --git a/paw/stos/head/arch/amd64/mcb.h b/paw/stos/head/arch/amd64/mcb.h
index 3722867..0357c50 100644
--- a/paw/stos/head/arch/amd64/mcb.h
+++ b/paw/stos/head/arch/amd64/mcb.h
@@ -18,11 +18,13 @@
  * @ModelId:    Processor model ID
  * @FamilyId:   Processor family ID
  * @LapicBase:  Local APIC MMIO base
+ * @HasX2Apic:  If set, x2APIC is supported
  */
 typedef struct {
     UCHAR ModelId;
     USHORT FamilyId : 12;
     VOID *LapicBase;
+    UCHAR HasX2Apic : 1;
 } MCB;
 
 #endif  /* !_MACHINE_MCB_H_ */