Index: head/sys/conf/files.powerpc
===================================================================
--- head/sys/conf/files.powerpc
+++ head/sys/conf/files.powerpc
@@ -192,6 +192,7 @@
 powerpc/powernv/opal_console.c	optional	powernv
 powerpc/powernv/opal_dev.c	optional	powernv
 powerpc/powernv/opal_flash.c	optional	powernv
+powerpc/powernv/opal_hmi.c	optional	powernv
 powerpc/powernv/opal_i2c.c	optional	iicbus fdt powernv
 powerpc/powernv/opal_i2cm.c	optional	iicbus fdt powernv
 powerpc/powernv/opal_pci.c	optional	powernv pci
Index: head/sys/powerpc/include/spr.h
===================================================================
--- head/sys/powerpc/include/spr.h
+++ head/sys/powerpc/include/spr.h
@@ -242,6 +242,8 @@
 #define	  LPCR_PECE_ME            (1ULL << 12) /* Machine Check and Hypervisor */
                                                /* Maintenance exceptions */
 #define	SPR_LPID		0x13f	/* Logical Partitioning Control */
+#define	SPR_HMER		0x150	/* Hypervisor Maintenance Exception Register */
+#define	SPR_HMEER		0x151	/* Hypervisor Maintenance Exception Enable Register */
 
 #define	SPR_PTCR		0x1d0	/* Partition Table Control Register */
 #define	SPR_SPEFSCR		0x200	/* ..8 Signal Processing Engine FSCR. */
Index: head/sys/powerpc/include/trap.h
===================================================================
--- head/sys/powerpc/include/trap.h
+++ head/sys/powerpc/include/trap.h
@@ -153,6 +153,7 @@
 #ifndef LOCORE
 struct	trapframe;
 struct	pcb;
+extern int	(*hmi_handler)(struct trapframe *);
 void    trap(struct trapframe *);
 int	ppc_instr_emulate(struct trapframe *, struct pcb *);
 #endif
Index: head/sys/powerpc/powernv/opal.h
===================================================================
--- head/sys/powerpc/powernv/opal.h
+++ head/sys/powerpc/powernv/opal.h
@@ -71,8 +71,10 @@
 #define	OPAL_PCI_MAP_PE_DMA_WINDOW_REAL	45
 #define	OPAL_RETURN_CPU			69
 #define	OPAL_REINIT_CPUS		70
+#define	OPAL_CHECK_TOKEN		80
 #define	OPAL_CHECK_ASYNC_COMPLETION	86
 #define	OPAL_SENSOR_READ		88
+#define	OPAL_HANDLE_HMI			98
 #define	OPAL_IPMI_SEND			107
 #define	OPAL_IPMI_RECV			108
 #define	OPAL_I2C_REQUEST		109
@@ -102,6 +104,7 @@
 #define	OPAL_SENSOR_GROUP_CLEAR		156
 #define	OPAL_SENSOR_READ_U64		162
 #define	OPAL_SENSOR_GROUP_ENABLE	163
+#define	OPAL_HANDLE_HMI2		166
 
 /* For OPAL_PCI_SET_PE */
 #define	OPAL_UNMAP_PE			0
@@ -133,6 +136,15 @@
 #define	OPAL_EMPTY			-16
 #define	OPAL_XIVE_PROVISIONING		-31
 #define	OPAL_XIVE_FREE_ACTIVE		-32
+
+#define	OPAL_TOKEN_ABSENT		0
+#define	OPAL_TOKEN_PRESENT		1
+
+#define	OPAL_HMI_FLAGS_TB_RESYNC	(1ull << 0)
+#define	OPAL_HMI_FLAGS_DEC_LOST		(1ull << 1)
+#define	OPAL_HMI_FLAGS_HDEC_LOST	(1ull << 2)
+#define	OPAL_HMI_FLAGS_TOD_TB_FAIL	(1ull << 3)
+#define	OPAL_HMI_FLAGS_NEW_EVENT	(1ull << 63)
 
 #define	OPAL_XIVE_XICS_MODE_EMU	0
 #define	OPAL_XIVE_XICS_MODE_EXP	1
Index: head/sys/powerpc/powernv/opal_hmi.c
===================================================================
--- head/sys/powerpc/powernv/opal_hmi.c
+++ head/sys/powerpc/powernv/opal_hmi.c
@@ -0,0 +1,97 @@
+/*-
+ * Copyright (c) 2019 Justin Hibbits
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/types.h>
+#include <sys/kernel.h>
+#include <sys/systm.h>
+
+#include <vm/vm.h>
+#include <vm/pmap.h>
+
+#include <machine/spr.h>
+#include <machine/trap.h>
+#include "opal.h"
+
+static int
+opal_hmi_handler2(struct trapframe *frame)
+{
+	int64_t flags;
+	int err;
+
+	err = opal_call(OPAL_HANDLE_HMI2, vtophys(&flags));
+
+	/* XXX: At some point, handle the flags outvar. */
+	if (err == OPAL_SUCCESS) {
+		mtspr(SPR_HMER, 0);
+		return (0);
+	}
+
+	printf("HMI handler failed!  OPAL error code: %d\n", err);
+
+	return (-1);
+}
+
+static int
+opal_hmi_handler(struct trapframe *frame)
+{
+	int err;
+
+	err = opal_call(OPAL_HANDLE_HMI);
+
+	if (err == OPAL_SUCCESS) {
+		mtspr(SPR_HMER, 0);
+		return (0);
+	}
+
+	printf("HMI handler failed!  OPAL error code: %d\n", err);
+
+	return (-1);
+}
+
+static void
+opal_setup_hmi(void *data)
+{
+	/* This only works for OPAL, so first make sure we have it. */
+	if (opal_check() != 0)
+		return;
+
+	if (opal_call(OPAL_CHECK_TOKEN, OPAL_HANDLE_HMI2) == OPAL_TOKEN_PRESENT)
+		hmi_handler = opal_hmi_handler2;
+	else if (opal_call(OPAL_CHECK_TOKEN, OPAL_HANDLE_HMI) == OPAL_TOKEN_PRESENT)
+		hmi_handler = opal_hmi_handler;
+	else {
+		printf("Warning: No OPAL HMI handler found.\n");
+		return;
+	}
+
+	if (bootverbose)
+		printf("Installed OPAL HMI handler.\n");
+}
+
+SYSINIT(opal_setup_hmi, SI_SUB_HYPERVISOR, SI_ORDER_ANY, opal_setup_hmi, NULL);
Index: head/sys/powerpc/powerpc/interrupt.c
===================================================================
--- head/sys/powerpc/powerpc/interrupt.c
+++ head/sys/powerpc/powerpc/interrupt.c
@@ -32,6 +32,7 @@
  */
 
 #include "opt_hwpmc_hooks.h"
+#include "opt_platform.h"
 
 #include <sys/cdefs.h>                  /* RCS ID & Copyright macro defns */
 
@@ -68,6 +69,10 @@
 
 #include "pic_if.h"
 
+#ifdef POWERNV
+int (*hmi_handler)(struct trapframe *);
+#endif
+
 /*
  * A very short dispatch, to try and maximise assembler code use
  * between all exception types. Maybe 'true' interrupts should go
@@ -117,6 +122,13 @@
 			pmc_hook(PCPU_GET(curthread), PMC_FN_USER_CALLCHAIN, framep);
 		critical_exit();
 		break;
+#endif
+
+#ifdef POWERNV
+	case EXC_HMI:
+		if (hmi_handler != 0 && hmi_handler(framep) == 0)
+			break;
+		/* If no handler, or failure to handle, just drop to trap. */
 #endif
 
 	default: