Index: sys/dev/iommu/iommu.h
===================================================================
--- sys/dev/iommu/iommu.h
+++ sys/dev/iommu/iommu.h
@@ -109,6 +109,7 @@
 	struct iommu_map_entries_tailq unload_entries; /* (d) Entries to
 							 unload */
 	struct iommu_gas_entries_tree rb_root; /* (d) */
+	struct iommu_map_entry *start_gap;     /* (d) */
 	iommu_gaddr_t end;		/* (c) Highest address + 1 in
 					   the guest AS */
 	struct iommu_map_entry *first_place, *last_place; /* (d) */
Index: sys/dev/iommu/iommu_gas.c
===================================================================
--- sys/dev/iommu/iommu_gas.c
+++ sys/dev/iommu/iommu_gas.c
@@ -210,6 +210,12 @@
 iommu_gas_rb_remove(struct iommu_domain *domain, struct iommu_map_entry *entry)
 {
 
+	/*
+	 * The predecessor to the removed entry may become the new greatest
+	 * lower bound on free gaps.
+	 */
+	if (entry->end <= domain->start_gap->end)
+		domain->start_gap = iommu_gas_entries_tree_RB_PREV(entry);
 	RB_REMOVE(iommu_gas_entries_tree, &domain->rb_root, entry);
 }
 
@@ -253,6 +259,7 @@
 	begin->flags = IOMMU_MAP_ENTRY_PLACE | IOMMU_MAP_ENTRY_UNMAPPED;
 	RB_INSERT_PREV(iommu_gas_entries_tree, &domain->rb_root, end, begin);
 
+	domain->start_gap = begin;
 	domain->first_place = begin;
 	domain->last_place = end;
 	domain->flags |= IOMMU_DOMAIN_GAS_INITED;
@@ -369,6 +376,9 @@
 	entry->start = start;
 	entry->end = start + roundup2(size + offset, IOMMU_PAGE_SIZE);
 	entry->flags = IOMMU_MAP_ENTRY_MAP;
+	/* The new entry may be a new, greater lower bound on free gaps. */
+	if (start <= a->domain->start_gap->end + 3 * IOMMU_PAGE_SIZE)
+		a->domain->start_gap = entry;
 	return (true);
 }
 
@@ -418,12 +428,9 @@
 	 * range.
 	 */
 	domain = a->domain;
-	curr = RB_ROOT(&domain->rb_root);
-	first = NULL;
-	while (curr != NULL && curr->free_down >= min_free) {
-		first = curr;
-		curr = RB_LEFT(curr, rb_entry);
-	}
+	first = domain->start_gap;
+	while (first != NULL && first->free_down < min_free)
+		first = RB_PARENT(first, rb_entry);
 
 	/*
 	 * Walk the big-enough ranges until one satisfies alignment