stos/amd64: lapic: Detect if x2APIC mode is present

Signed-off-by: Chloe M. <chloe@mensia.org>

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();