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
@@ -298,7 +298,7 @@
 
 	new_channel->ch_sigevt = hyperv_dmamem_alloc(
 	    bus_get_dma_tag(sc->vmbus_dev),
-	    HYPERCALL_SIGEVTIN_ALIGN, 0, sizeof(struct hypercall_sigevt_in),
+	    HYPERCALL_PARAM_ALIGN, 0, sizeof(struct hypercall_sigevt_in),
 	    &new_channel->ch_sigevt_dma, BUS_DMA_WAITOK | BUS_DMA_ZERO);
 	if (new_channel->ch_sigevt == NULL) {
 		device_printf(sc->vmbus_dev, "sigevt alloc failed\n");
Index: head/sys/dev/hyperv/vmbus/hyperv_reg.h
===================================================================
--- head/sys/dev/hyperv/vmbus/hyperv_reg.h
+++ head/sys/dev/hyperv/vmbus/hyperv_reg.h
@@ -153,13 +153,22 @@
 /*
  * Hypercall input parameters
  */
+#define HYPERCALL_PARAM_ALIGN		8
+#if 0
+/*
+ * XXX
+ * <<Hypervisor Top Level Functional Specification 4.0b>> requires
+ * input parameters size to be multiple of 8, however, many post
+ * message input parameters do _not_ meet this requirement.
+ */
+#define HYPERCALL_PARAM_SIZE_ALIGN	8
+#endif
 
 /*
  * HYPERCALL_POST_MESSAGE
  */
 #define HYPERCALL_POSTMSGIN_DSIZE_MAX	240
 #define HYPERCALL_POSTMSGIN_SIZE	256
-#define HYPERCALL_POSTMSGIN_ALIGN	8
 
 struct hypercall_postmsg_in {
 	uint32_t	hc_connid;
@@ -173,8 +182,6 @@
 /*
  * HYPERCALL_SIGNAL_EVENT
  */
-#define HYPERCALL_SIGEVTIN_ALIGN	8
-
 struct hypercall_sigevt_in {
 	uint32_t	hc_connid;
 	uint16_t	hc_evtflag_ofs;
Index: head/sys/dev/hyperv/vmbus/vmbus.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus.c
+++ head/sys/dev/hyperv/vmbus/vmbus.c
@@ -130,7 +130,7 @@
 	mh = malloc(sizeof(*mh), M_DEVBUF, M_WAITOK | M_ZERO);
 
 	mh->mh_inprm = hyperv_dmamem_alloc(parent_dtag,
-	    HYPERCALL_POSTMSGIN_ALIGN, 0, HYPERCALL_POSTMSGIN_SIZE,
+	    HYPERCALL_PARAM_ALIGN, 0, HYPERCALL_POSTMSGIN_SIZE,
 	    &mh->mh_inprm_dma, BUS_DMA_WAITOK);
 	if (mh->mh_inprm == NULL) {
 		free(mh, M_DEVBUF);