Index: head/sys/x86/acpica/madt.c
===================================================================
--- head/sys/x86/acpica/madt.c
+++ head/sys/x86/acpica/madt.c
@@ -212,6 +212,14 @@
 		}
 	}
 
+	/*
+	 * Truncate max_apic_id if not in x2APIC mode. Some structures
+	 * will already be allocated with the previous max_apic_id, but
+	 * at least we can prevent wasting more memory elsewhere.
+	 */
+	if (!x2apic_mode)
+		max_apic_id = min(max_apic_id, xAPIC_MAX_APIC_ID);
+
 	madt = pmap_mapbios(madt_physaddr, madt_length);
 	lapics = malloc(sizeof(*lapics) * (max_apic_id + 1), M_MADT,
 	    M_WAITOK | M_ZERO);
@@ -250,7 +258,7 @@
 		panic("Using MADT but ACPI doesn't work");
 	}
 
-	ioapics = malloc(sizeof(*ioapics) * (MAX_APIC_ID + 1), M_MADT,
+	ioapics = malloc(sizeof(*ioapics) * (IOAPIC_MAX_ID + 1), M_MADT,
 	    M_WAITOK | M_ZERO);
 
 	/* First, we run through adding I/O APIC's. */
@@ -277,7 +285,7 @@
 	}
 
 	/* Third, we register all the I/O APIC's. */
-	for (i = 0; i <= MAX_APIC_ID; i++)
+	for (i = 0; i <= IOAPIC_MAX_ID; i++)
 		if (ioapics[i].io_apic != NULL)
 			ioapic_register(ioapics[i].io_apic);
 
@@ -408,7 +416,7 @@
 			    "MADT: Found IO APIC ID %u, Interrupt %u at %p\n",
 			    apic->Id, apic->GlobalIrqBase,
 			    (void *)(uintptr_t)apic->Address);
-		if (apic->Id > MAX_APIC_ID)
+		if (apic->Id > IOAPIC_MAX_ID)
 			panic("%s: I/O APIC ID %u too high", __func__,
 			    apic->Id);
 		if (ioapics[apic->Id].io_apic != NULL)
@@ -501,7 +509,7 @@
 	int i, best;
 
 	best = -1;
-	for (i = 0; i <= MAX_APIC_ID; i++) {
+	for (i = 0; i <= IOAPIC_MAX_ID; i++) {
 		if (ioapics[i].io_apic == NULL ||
 		    ioapics[i].io_vector > intr)
 			continue;
Index: head/sys/x86/include/apicvar.h
===================================================================
--- head/sys/x86/include/apicvar.h
+++ head/sys/x86/include/apicvar.h
@@ -74,8 +74,12 @@
  * I/O device!
  */
 
-#define	MAX_APIC_ID	0xfe
-#define	APIC_ID_ALL	0xff
+#define	xAPIC_MAX_APIC_ID	0xfe
+#define	xAPIC_ID_ALL		0xff
+#define	MAX_APIC_ID		0x200
+#define	APIC_ID_ALL		0xffffffff
+
+#define	IOAPIC_MAX_ID		xAPIC_MAX_APIC_ID
 
 /* I/O Interrupts are used for external devices such as ISA, PCI, etc. */
 #define	APIC_IO_INTS	(IDT_IO_INTS + 16)
Index: head/sys/x86/x86/mp_x86.c
===================================================================
--- head/sys/x86/x86/mp_x86.c
+++ head/sys/x86/x86/mp_x86.c
@@ -137,6 +137,10 @@
 struct cpu_info *cpu_info;
 int *apic_cpuids;
 int cpu_apic_ids[MAXCPU];
+_Static_assert(MAXCPU <= MAX_APIC_ID,
+    "MAXCPU cannot be larger that MAX_APIC_ID");
+_Static_assert(xAPIC_MAX_APIC_ID <= MAX_APIC_ID,
+    "xAPIC_MAX_APIC_ID cannot be larger that MAX_APIC_ID");
 
 /* Holds pending bitmap based IPIs per CPU */
 volatile u_int cpu_ipi_pending[MAXCPU];
@@ -830,18 +834,18 @@
 		panic("SMP: APIC ID %d too high", apic_id);
 		return;
 	}
-	KASSERT(cpu_info[apic_id].cpu_present == 0, ("CPU %d added twice",
+	KASSERT(cpu_info[apic_id].cpu_present == 0, ("CPU %u added twice",
 	    apic_id));
 	cpu_info[apic_id].cpu_present = 1;
 	if (boot_cpu) {
 		KASSERT(boot_cpu_id == -1,
-		    ("CPU %d claims to be BSP, but CPU %d already is", apic_id,
+		    ("CPU %u claims to be BSP, but CPU %u already is", apic_id,
 		    boot_cpu_id));
 		boot_cpu_id = apic_id;
 		cpu_info[apic_id].cpu_bsp = 1;
 	}
 	if (bootverbose)
-		printf("SMP: Added CPU %d (%s)\n", apic_id, boot_cpu ? "BSP" :
+		printf("SMP: Added CPU %u (%s)\n", apic_id, boot_cpu ? "BSP" :
 		    "AP");
 }
 
Index: head/sys/x86/x86/mptable.c
===================================================================
--- head/sys/x86/x86/mptable.c
+++ head/sys/x86/x86/mptable.c
@@ -159,7 +159,7 @@
 static mpfps_t mpfps;
 static mpcth_t mpct;
 static ext_entry_ptr mpet;
-static void *ioapics[MAX_APIC_ID + 1];
+static void *ioapics[IOAPIC_MAX_ID + 1];
 static bus_datum *busses;
 static int mptable_nioapics, mptable_nbusses, mptable_maxbusid;
 static int pci0 = -1;
@@ -393,7 +393,7 @@
 	mptable_parse_ints();
 
 	/* Fourth, we register all the I/O APIC's. */
-	for (i = 0; i <= MAX_APIC_ID; i++)
+	for (i = 0; i <= IOAPIC_MAX_ID; i++)
 		if (ioapics[i] != NULL)
 			ioapic_register(ioapics[i]);
 
@@ -589,7 +589,7 @@
 		apic = (io_apic_entry_ptr)entry;
 		if (!(apic->apic_flags & IOAPICENTRY_FLAG_EN))
 			break;
-		if (apic->apic_id > MAX_APIC_ID)
+		if (apic->apic_id > IOAPIC_MAX_ID)
 			panic("%s: I/O APIC ID %d too high", __func__,
 			    apic->apic_id);
 		if (ioapics[apic->apic_id] != NULL)
@@ -736,7 +736,7 @@
 			return;
 		}
 	}
-	if (apic_id > MAX_APIC_ID) {
+	if (apic_id > IOAPIC_MAX_ID) {
 		printf("MPTable: Ignoring interrupt entry for ioapic%d\n",
 		    intr->dst_apic_id);
 		return;