diff --git a/sys/amd64/include/pcpu.h b/sys/amd64/include/pcpu.h
index 95a647747a79..4ca9d5588e5f 100644
--- a/sys/amd64/include/pcpu.h
+++ b/sys/amd64/include/pcpu.h
@@ -36,6 +36,13 @@
 #endif
 
 #define	PC_PTI_STACK_SZ	16
+
+struct monitorbuf {
+	int idle_state;		/* Used by cpu_idle_mwait. */
+	char padding[128 - (1 * sizeof(int))];
+};
+_Static_assert(sizeof(struct monitorbuf) == 128, "2x cache line");
+
 /*
  * The SMP parts are setup in pmap.c and locore.s for the BSP, and
  * mp_machdep.c sets up the data for the AP's to "see" when they awake.
@@ -44,7 +51,7 @@
  * other processors"
  */
 #define	PCPU_MD_FIELDS							\
-	char	pc_monitorbuf[128] __aligned(128); /* cache line */	\
+	struct monitorbuf pc_monitorbuf __aligned(128);	/* cache line */\
 	struct	pcpu *pc_prvspace;	/* Self-reference */		\
 	struct	pmap *pc_curpmap;					\
 	struct	amd64tss *pc_tssp;	/* TSS segment active on CPU */	\
diff --git a/sys/i386/include/pcpu.h b/sys/i386/include/pcpu.h
index 50864d7d3648..3ae8e7e14422 100644
--- a/sys/i386/include/pcpu.h
+++ b/sys/i386/include/pcpu.h
@@ -41,6 +41,12 @@
 #include <sys/_lock.h>
 #include <sys/_mutex.h>
 
+struct monitorbuf {
+	int idle_state;		/* Used by cpu_idle_mwait. */
+	char padding[128 - (1 * sizeof(int))];
+};
+_Static_assert(sizeof(struct monitorbuf) == 128, "2x cache line");
+
 /*
  * The SMP parts are setup in pmap.c and machdep.c for the BSP, and
  * pmap.c and mp_machdep.c sets up the data for the AP's to "see" when
@@ -50,7 +56,7 @@
  */
 
 #define	PCPU_MD_FIELDS							\
-	char	pc_monitorbuf[128] __aligned(128); /* cache line */	\
+	struct monitorbuf pc_monitorbuf __aligned(128);	/* cache line */\
 	struct	pcpu *pc_prvspace;	/* Self-reference */		\
 	struct	pmap *pc_curpmap;					\
 	struct	segment_descriptor pc_common_tssd;			\
diff --git a/sys/x86/x86/cpu_machdep.c b/sys/x86/x86/cpu_machdep.c
index eb4d5824c7e3..71925c38560d 100644
--- a/sys/x86/x86/cpu_machdep.c
+++ b/sys/x86/x86/cpu_machdep.c
@@ -164,7 +164,7 @@ acpi_cpu_idle_mwait(uint32_t mwait_hint)
 	 * but all Intel CPUs provide hardware coordination.
 	 */
 
-	state = (int *)PCPU_PTR(monitorbuf);
+	state = &PCPU_PTR(monitorbuf)->idle_state;
 	KASSERT(atomic_load_int(state) == STATE_SLEEPING,
 	    ("cpu_mwait_cx: wrong monitorbuf state"));
 	atomic_store_int(state, STATE_MWAIT);
@@ -422,7 +422,7 @@ cpu_idle_acpi(sbintime_t sbt)
 {
 	int *state;
 
-	state = (int *)PCPU_PTR(monitorbuf);
+	state = &PCPU_PTR(monitorbuf)->idle_state;
 	atomic_store_int(state, STATE_SLEEPING);
 
 	/* See comments in cpu_idle_hlt(). */
@@ -441,7 +441,7 @@ cpu_idle_hlt(sbintime_t sbt)
 {
 	int *state;
 
-	state = (int *)PCPU_PTR(monitorbuf);
+	state = &PCPU_PTR(monitorbuf)->idle_state;
 	atomic_store_int(state, STATE_SLEEPING);
 
 	/*
@@ -473,7 +473,7 @@ cpu_idle_mwait(sbintime_t sbt)
 {
 	int *state;
 
-	state = (int *)PCPU_PTR(monitorbuf);
+	state = &PCPU_PTR(monitorbuf)->idle_state;
 	atomic_store_int(state, STATE_MWAIT);
 
 	/* See comments in cpu_idle_hlt(). */
@@ -498,7 +498,7 @@ cpu_idle_spin(sbintime_t sbt)
 	int *state;
 	int i;
 
-	state = (int *)PCPU_PTR(monitorbuf);
+	state = &PCPU_PTR(monitorbuf)->idle_state;
 	atomic_store_int(state, STATE_RUNNING);
 
 	/*
@@ -598,9 +598,11 @@ SYSCTL_INT(_machdep, OID_AUTO, idle_apl31, CTLFLAG_RW,
 int
 cpu_idle_wakeup(int cpu)
 {
+	struct monitorbuf *mb;
 	int *state;
 
-	state = (int *)pcpu_find(cpu)->pc_monitorbuf;
+	mb = &pcpu_find(cpu)->pc_monitorbuf;
+	state = &mb->idle_state;
 	switch (atomic_load_int(state)) {
 	case STATE_SLEEPING:
 		return (0);