diff --git a/sys/arm/arm/pmu_acpi.c b/sys/arm/arm/pmu_acpi.c
--- a/sys/arm/arm/pmu_acpi.c
+++ b/sys/arm/arm/pmu_acpi.c
@@ -72,7 +72,7 @@
 
 	for (i = 0; i < MAXCPU; i++) {
 		pcpu = pcpu_find(i);
-		if (pcpu != NULL && pcpu->pc_mpidr == intr->ArmMpidr) {
+		if (pcpu != NULL && PCPU_GET_MPIDR(pcpu) == intr->ArmMpidr) {
 			cpuid = i;
 			break;
 		}
diff --git a/sys/arm/arm/pmu_fdt.c b/sys/arm/arm/pmu_fdt.c
--- a/sys/arm/arm/pmu_fdt.c
+++ b/sys/arm/arm/pmu_fdt.c
@@ -105,7 +105,7 @@
 
 	for (i = 0; i < MAXCPU; i++) {
 		pcpu = pcpu_find(i);
-		if (pcpu != NULL && pcpu->pc_mpidr == mpidr) {
+		if (pcpu != NULL && PCPU_GET_MPIDR(pcpu) == mpidr) {
 			irq->cpuid = i;
 			return (0);
 		}
diff --git a/sys/arm/include/pcpu.h b/sys/arm/include/pcpu.h
--- a/sys/arm/include/pcpu.h
+++ b/sys/arm/include/pcpu.h
@@ -139,6 +139,8 @@
 #define	PCPU_PTR(member)	(&get_pcpu()->pc_ ## member)
 #define	PCPU_SET(member,value)	(get_pcpu()->pc_ ## member = (value))
 
+#define	PCPU_GET_MPIDR(pc)	((pc)->pc_mpidr)
+
 void pcpu0_init(void);
 #endif	/* _KERNEL */
 
diff --git a/sys/arm64/arm64/machdep.c b/sys/arm64/arm64/machdep.c
--- a/sys/arm64/arm64/machdep.c
+++ b/sys/arm64/arm64/machdep.c
@@ -300,7 +300,8 @@
 {
 
 	pcpu->pc_acpi_id = 0xffffffff;
-	pcpu->pc_mpidr = 0xffffffff;
+	pcpu->pc_mpidr_low = 0xffffffff;
+	pcpu->pc_mpidr_high = 0xffffffff;
 }
 
 void
diff --git a/sys/arm64/arm64/mp_machdep.c b/sys/arm64/arm64/mp_machdep.c
--- a/sys/arm64/arm64/mp_machdep.c
+++ b/sys/arm64/arm64/mp_machdep.c
@@ -153,7 +153,7 @@
 is_boot_cpu(uint64_t target_cpu)
 {
 
-	return (cpuid_to_pcpu[0]->pc_mpidr == (target_cpu & CPU_AFF_MASK));
+	return (PCPU_GET_MPIDR(cpuid_to_pcpu[0]) == (target_cpu & CPU_AFF_MASK));
 }
 
 static void
@@ -207,7 +207,7 @@
 {
 	struct pcpu *pcpup;
 	pmap_t pmap0;
-	u_int mpidr;
+	uint64_t mpidr;
 
 	ptrauth_mp_start(cpu);
 
@@ -218,10 +218,10 @@
 	 */
 	mpidr = READ_SPECIALREG(mpidr_el1) & CPU_AFF_MASK;
 	if (cpu >= MAXCPU || cpuid_to_pcpu[cpu] == NULL ||
-	    cpuid_to_pcpu[cpu]->pc_mpidr != mpidr) {
+	    PCPU_GET_MPIDR(cpuid_to_pcpu[cpu]) != mpidr) {
 		for (cpu = 0; cpu < mp_maxid; cpu++)
 			if (cpuid_to_pcpu[cpu] != NULL &&
-			    cpuid_to_pcpu[cpu]->pc_mpidr == mpidr)
+			    PCPU_GET_MPIDR(cpuid_to_pcpu[cpu]) == mpidr)
 				break;
 		if ( cpu >= MAXCPU)
 			panic("MPIDR for this CPU is not in pcpu table");
@@ -517,7 +517,8 @@
 
 	pcpup = (struct pcpu *)pcpu_mem;
 	pcpu_init(pcpup, cpuid, sizeof(struct pcpu));
-	pcpup->pc_mpidr = target_cpu & CPU_AFF_MASK;
+	pcpup->pc_mpidr_low = target_cpu & CPU_AFF_MASK;
+	pcpup->pc_mpidr_high = (target_cpu & CPU_AFF_MASK) >> 32;
 
 	dpcpu[cpuid - 1] = (void *)(pcpup + 1);
 	dpcpu_init(dpcpu[cpuid - 1], cpuid);
@@ -688,11 +689,15 @@
 void
 cpu_mp_start(void)
 {
+	uint64_t mpidr;
+
 	mtx_init(&ap_boot_mtx, "ap boot", NULL, MTX_SPIN);
 
 	/* CPU 0 is always boot CPU. */
 	CPU_SET(0, &all_cpus);
-	cpuid_to_pcpu[0]->pc_mpidr = READ_SPECIALREG(mpidr_el1) & CPU_AFF_MASK;
+	mpidr = READ_SPECIALREG(mpidr_el1) & CPU_AFF_MASK;
+	cpuid_to_pcpu[0]->pc_mpidr_low = mpidr;
+	cpuid_to_pcpu[0]->pc_mpidr_high = mpidr >> 32;
 
 	switch(arm64_bus_method) {
 #ifdef DEV_ACPI
diff --git a/sys/arm64/include/pcpu.h b/sys/arm64/include/pcpu.h
--- a/sys/arm64/include/pcpu.h
+++ b/sys/arm64/include/pcpu.h
@@ -48,8 +48,10 @@
 	struct pmap *pc_curpmap;					\
 	struct pmap *pc_curvmpmap;					\
 	u_int	pc_bcast_tlbi_workaround;				\
-	u_int	pc_mpidr;	/* stored MPIDR value */		\
-	char __pad[201]
+	/* Store as two u_int values to preserve KBI */			\
+	u_int	pc_mpidr_low;	/* lower MPIDR 32 bits */		\
+	u_int	pc_mpidr_high;	/* upper MPIDR 32 bits */		\
+	char __pad[197]
 
 #ifdef _KERNEL
 
@@ -83,6 +85,9 @@
 #define	PCPU_PTR(member)	(&pcpup->pc_ ## member)
 #define	PCPU_SET(member,value)	(pcpup->pc_ ## member = (value))
 
+#define	PCPU_GET_MPIDR(pc)	\
+    ((((uint64_t)((pc)->pc_mpidr_high)) << 32) | ((pc)->pc_mpidr_low))
+
 #endif	/* _KERNEL */
 
 #endif	/* !_MACHINE_PCPU_H_ */