diff --git a/sys/arm64/arm64/gicv3_its.c b/sys/arm64/arm64/gicv3_its.c
--- a/sys/arm64/arm64/gicv3_its.c
+++ b/sys/arm64/arm64/gicv3_its.c
@@ -1483,7 +1483,8 @@
 	ctx = iommu_get_dev_ctx(child);
 	if (ctx == NULL)
 		return (ENXIO);
-	error = iommu_map_msi(ctx, PAGE_SIZE, GITS_TRANSLATER,
+	/* Map the page containing the GITS_TRANSLATER register. */
+	error = iommu_map_msi(ctx, PAGE_SIZE, 0,
 	    IOMMU_MAP_ENTRY_WRITE, IOMMU_MF_CANWAIT, &sc->ma);
 	*domain = iommu_get_ctx_domain(ctx);
 
diff --git a/sys/dev/iommu/busdma_iommu.c b/sys/dev/iommu/busdma_iommu.c
--- a/sys/dev/iommu/busdma_iommu.c
+++ b/sys/dev/iommu/busdma_iommu.c
@@ -594,8 +594,7 @@
 		if (seg + 1 < tag->common.nsegments)
 			gas_flags |= IOMMU_MF_CANSPLIT;
 
-		error = iommu_map(domain, &tag->common,
-		    round_page(offset + buflen1),
+		error = iommu_map(domain, &tag->common, buflen1,
 		    offset, e_flags, gas_flags, ma + idx, &entry);
 		if (error != 0)
 			break;
diff --git a/sys/dev/iommu/iommu_gas.c b/sys/dev/iommu/iommu_gas.c
--- a/sys/dev/iommu/iommu_gas.c
+++ b/sys/dev/iommu/iommu_gas.c
@@ -364,7 +364,8 @@
 	 * The page sized gap is created between consequent
 	 * allocations to ensure that out-of-bounds accesses fault.
 	 */
-	a->entry->end = a->entry->start + a->size;
+	a->entry->end = a->entry->start +
+	    roundup2(a->size + a->offset, IOMMU_PAGE_SIZE);
 
 	found = iommu_gas_rb_insert(a->domain, a->entry);
 	KASSERT(found, ("found dup %p start %jx size %jx",
@@ -381,7 +382,8 @@
 	 * If the subtree doesn't have free space for the requested allocation
 	 * plus two guard pages, give up.
 	 */
-	if (entry->free_down < a->size + a->offset + 2 * IOMMU_PAGE_SIZE)
+	if (entry->free_down < 2 * IOMMU_PAGE_SIZE +
+	    roundup2(a->size + a->offset, IOMMU_PAGE_SIZE))
 		return (ENOMEM);
 	if (entry->first >= a->common->lowaddr)
 		return (ENOMEM);
@@ -415,7 +417,8 @@
 	 * If the subtree doesn't have free space for the requested allocation
 	 * plus two guard pages, give up.
 	 */
-	if (entry->free_down < a->size + a->offset + 2 * IOMMU_PAGE_SIZE)
+	if (entry->free_down < 2 * IOMMU_PAGE_SIZE +
+	    roundup2(a->size + a->offset, IOMMU_PAGE_SIZE))
 		return (ENOMEM);
 	if (entry->last < a->common->highaddr)
 		return (ENOMEM);
@@ -450,7 +453,6 @@
 
 	IOMMU_DOMAIN_ASSERT_LOCKED(domain);
 	KASSERT(entry->flags == 0, ("dirty entry %p %p", domain, entry));
-	KASSERT((size & IOMMU_PAGE_MASK) == 0, ("size %jx", (uintmax_t)size));
 
 	a.domain = domain;
 	a.size = size;