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
@@ -305,7 +305,7 @@
 
 	a->entry->start = roundup2(beg + IOMMU_PAGE_SIZE,
 	    a->common->alignment);
-	if (a->entry->start + a->size > maxaddr)
+	if (a->entry->start + a->offset + a->size > maxaddr)
 		return (false);
 
 	/* IOMMU_PAGE_SIZE to create gap after new entry. */