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
@@ -387,18 +387,18 @@
 		return (ENOMEM);
 	if (entry->first >= a->common->lowaddr)
 		return (ENOMEM);
-	child = RB_RIGHT(entry, rb_entry);
+	child = RB_LEFT(entry, rb_entry);
 	if (child != NULL && 0 == iommu_gas_lowermatch(a, child))
 		return (0);
-	if (child != NULL && entry->end < a->common->lowaddr &&
-	    iommu_gas_match_one(a, entry->end, child->first,
+	if (child != NULL && child->last < a->common->lowaddr &&
+	    iommu_gas_match_one(a, child->last, entry->start,
 	    a->common->lowaddr)) {
 		iommu_gas_match_insert(a);
 		return (0);
 	}
-	child = RB_LEFT(entry, rb_entry);
-	if (child != NULL && child->last < a->common->lowaddr &&
-	    iommu_gas_match_one(a, child->last, entry->start,
+	child = RB_RIGHT(entry, rb_entry);
+	if (child != NULL && entry->end < a->common->lowaddr &&
+	    iommu_gas_match_one(a, entry->end, child->first,
 	    a->common->lowaddr)) {
 		iommu_gas_match_insert(a);
 		return (0);