Index: head/sys/dev/hyperv/vmbus/hv_channel_mgmt.c
===================================================================
--- head/sys/dev/hyperv/vmbus/hv_channel_mgmt.c
+++ head/sys/dev/hyperv/vmbus/hv_channel_mgmt.c
@@ -40,23 +40,29 @@
  * Internal functions
  */
 
-typedef void (*vmbus_msg_handler)(hv_vmbus_channel_msg_header *msg);
+typedef void (*vmbus_msg_handler)(const hv_vmbus_channel_msg_header *msg);
 
 typedef struct hv_vmbus_channel_msg_table_entry {
 	hv_vmbus_channel_msg_type    messageType;
-
 	vmbus_msg_handler   messageHandler;
 } hv_vmbus_channel_msg_table_entry;
 
-static void vmbus_channel_on_offer(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_offer_internal(void* context);
-static void vmbus_channel_on_open_result(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_offer_rescind(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_offer_rescind_internal(void* context);
-static void vmbus_channel_on_gpadl_created(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_gpadl_torndown(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_offers_delivered(hv_vmbus_channel_msg_header* hdr);
-static void vmbus_channel_on_version_response(hv_vmbus_channel_msg_header* hdr);
+static void	vmbus_channel_on_offer_internal(void *context);
+static void	vmbus_channel_on_offer_rescind_internal(void *context);
+
+static void	vmbus_channel_on_offer(const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_open_result(
+		    const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_offer_rescind(
+		    const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_gpadl_created(
+		    const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_gpadl_torndown(
+		    const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_offers_delivered(
+		    const hv_vmbus_channel_msg_header *hdr);
+static void	vmbus_channel_on_version_response(
+		    const hv_vmbus_channel_msg_header *hdr);
 
 /**
  * Channel message dispatch table
@@ -392,12 +398,12 @@
  * object to process the offer synchronously
  */
 static void
-vmbus_channel_on_offer(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_offer(const hv_vmbus_channel_msg_header *hdr)
 {
-	hv_vmbus_channel_offer_channel* offer;
-	hv_vmbus_channel_offer_channel* copied;
+	const hv_vmbus_channel_offer_channel *offer;
+	hv_vmbus_channel_offer_channel *copied;
 
-	offer = (hv_vmbus_channel_offer_channel*) hdr;
+	offer = (const hv_vmbus_channel_offer_channel *)hdr;
 
 	// copy offer data
 	copied = malloc(sizeof(*copied), M_DEVBUF, M_NOWAIT);
@@ -470,12 +476,12 @@
  * synchronously
  */
 static void
-vmbus_channel_on_offer_rescind(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_offer_rescind(const hv_vmbus_channel_msg_header *hdr)
 {
-	hv_vmbus_channel_rescind_offer*	rescind;
+	const hv_vmbus_channel_rescind_offer *rescind;
 	hv_vmbus_channel*		channel;
 
-	rescind = (hv_vmbus_channel_rescind_offer*) hdr;
+	rescind = (const hv_vmbus_channel_rescind_offer *)hdr;
 
 	channel = hv_vmbus_g_connection.channels[rescind->child_rel_id];
 	if (channel == NULL)
@@ -502,7 +508,8 @@
  * @brief Invoked when all offers have been delivered.
  */
 static void
-vmbus_channel_on_offers_delivered(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_offers_delivered(
+    const hv_vmbus_channel_msg_header *hdr __unused)
 {
 
 	mtx_lock(&vmbus_chwait_lock);
@@ -519,14 +526,14 @@
  * response and signal the requesting thread.
  */
 static void
-vmbus_channel_on_open_result(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_open_result(const hv_vmbus_channel_msg_header *hdr)
 {
-	hv_vmbus_channel_open_result*	result;
+	const hv_vmbus_channel_open_result *result;
 	hv_vmbus_channel_msg_info*	msg_info;
 	hv_vmbus_channel_msg_header*	requestHeader;
 	hv_vmbus_channel_open_channel*	openMsg;
 
-	result = (hv_vmbus_channel_open_result*) hdr;
+	result = (const hv_vmbus_channel_open_result *)hdr;
 
 	/*
 	 * Find the open msg, copy the result and signal/unblock the wait event
@@ -561,14 +568,14 @@
  * response and signal the requesting thread.
  */
 static void
-vmbus_channel_on_gpadl_created(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_gpadl_created(const hv_vmbus_channel_msg_header *hdr)
 {
-	hv_vmbus_channel_gpadl_created*		gpadl_created;
+	const hv_vmbus_channel_gpadl_created *gpadl_created;
 	hv_vmbus_channel_msg_info*		msg_info;
 	hv_vmbus_channel_msg_header*		request_header;
 	hv_vmbus_channel_gpadl_header*		gpadl_header;
 
-	gpadl_created = (hv_vmbus_channel_gpadl_created*) hdr;
+	gpadl_created = (const hv_vmbus_channel_gpadl_created *)hdr;
 
 	/* Find the establish msg, copy the result and signal/unblock
 	 * the wait event
@@ -603,14 +610,14 @@
  * response and signal the requesting thread
  */
 static void
-vmbus_channel_on_gpadl_torndown(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_gpadl_torndown(const hv_vmbus_channel_msg_header *hdr)
 {
-	hv_vmbus_channel_gpadl_torndown*	gpadl_torndown;
+	const hv_vmbus_channel_gpadl_torndown *gpadl_torndown;
 	hv_vmbus_channel_msg_info*		msg_info;
 	hv_vmbus_channel_msg_header*		requestHeader;
 	hv_vmbus_channel_gpadl_teardown*	gpadlTeardown;
 
-	gpadl_torndown = (hv_vmbus_channel_gpadl_torndown*)hdr;
+	gpadl_torndown = (const hv_vmbus_channel_gpadl_torndown *)hdr;
 
 	/*
 	 * Find the open msg, copy the result and signal/unblock the
@@ -648,14 +655,14 @@
  * response and signal the requesting thread.
  */
 static void
-vmbus_channel_on_version_response(hv_vmbus_channel_msg_header* hdr)
+vmbus_channel_on_version_response(const hv_vmbus_channel_msg_header *hdr)
 {
 	hv_vmbus_channel_msg_info*		msg_info;
 	hv_vmbus_channel_msg_header*		requestHeader;
 	hv_vmbus_channel_initiate_contact*	initiate;
-	hv_vmbus_channel_version_response*	versionResponse;
+	const hv_vmbus_channel_version_response *versionResponse;
 
-	versionResponse = (hv_vmbus_channel_version_response*)hdr;
+	versionResponse = (const hv_vmbus_channel_version_response *)hdr;
 
 	mtx_lock(&hv_vmbus_g_connection.channel_msg_lock);
 	TAILQ_FOREACH(msg_info, &hv_vmbus_g_connection.channel_msg_anchor,
@@ -841,14 +848,13 @@
 }
 
 void
-vmbus_chan_msgproc(struct vmbus_softc *sc, volatile struct vmbus_message *msg)
+vmbus_chan_msgproc(struct vmbus_softc *sc, const struct vmbus_message *msg)
 {
 	const hv_vmbus_channel_msg_table_entry *entry;
-	hv_vmbus_channel_msg_header *hdr;
+	const hv_vmbus_channel_msg_header *hdr;
 	hv_vmbus_channel_msg_type msg_type;
 
-	/* XXX: update messageHandler interface */
-	hdr = __DEVOLATILE(hv_vmbus_channel_msg_header *, msg->msg_data);
+	hdr = (const hv_vmbus_channel_msg_header *)msg->msg_data;
 	msg_type = hdr->message_type;
 
 	if (msg_type >= HV_CHANNEL_MESSAGE_COUNT) {
Index: head/sys/dev/hyperv/vmbus/vmbus.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus.c
+++ head/sys/dev/hyperv/vmbus/vmbus.c
@@ -86,7 +86,8 @@
 			break;
 		} else if (msg->msg_type == VMBUS_MSGTYPE_CHANNEL) {
 			/* Channel message */
-			vmbus_chan_msgproc(sc, msg);
+			vmbus_chan_msgproc(sc,
+			    __DEVOLATILE(const struct vmbus_message *, msg));
 		}
 
 		msg->msg_type = VMBUS_MSGTYPE_NONE;
Index: head/sys/dev/hyperv/vmbus/vmbus_var.h
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_var.h
+++ head/sys/dev/hyperv/vmbus/vmbus_var.h
@@ -102,7 +102,6 @@
 
 void	vmbus_et_intr(struct trapframe *);
 
-void	vmbus_chan_msgproc(struct vmbus_softc *,
-	    volatile struct vmbus_message *);
+void	vmbus_chan_msgproc(struct vmbus_softc *, const struct vmbus_message *);
 
 #endif	/* !_VMBUS_VAR_H_ */