diff --git a/share/man/man3/tree.3 b/share/man/man3/tree.3
--- a/share/man/man3/tree.3
+++ b/share/man/man3/tree.3
@@ -520,10 +520,26 @@
 and
 .Fn RB_NFIND
 macros can be used to find a particular element in the tree.
+.Pp
+The
+.Fn RB_FIND
+searches for the element which compares equal to the provided
+key element, with possible
+.Dv NULL
+result if no such element exists.
+.Pp
+The
+.Fn RB_NFIND
+searches for the least element which compares greater or equal
+to the provided key element (key is on the left side of comparision),
+again with possible
+.Dv NULL
+result if no such element exists.
 .Bd -literal -offset indent
-struct TYPE find, *res;
+struct TYPE find, *res, *resn;
 find.key = 30;
 res = RB_FIND(NAME, head, &find);
+resn = RB_NFIND(NAME, head, &find);
 .Ed
 .Pp
 The
diff --git a/sys/dev/iommu/iommu.h b/sys/dev/iommu/iommu.h
--- a/sys/dev/iommu/iommu.h
+++ b/sys/dev/iommu/iommu.h
@@ -171,6 +171,8 @@
     u_int flags);
 void iommu_gas_free_entry(struct iommu_map_entry *entry);
 void iommu_gas_free_space(struct iommu_map_entry *entry);
+void iommu_gas_remove(struct iommu_domain *domain, iommu_gaddr_t start,
+    iommu_gaddr_t size);
 int iommu_gas_map(struct iommu_domain *domain,
     const struct bus_dma_tag_common *common, iommu_gaddr_t size, int offset,
     u_int eflags, u_int flags, vm_page_t *ma, struct iommu_map_entry **res);
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
@@ -600,6 +600,163 @@
 	IOMMU_DOMAIN_UNLOCK(domain);
 }
 
+static struct iommu_map_entry *
+iommu_gas_clip(struct iommu_domain *domain, struct iommu_map_entry *entry,
+    iommu_gaddr_t clip, struct iommu_map_entry **r)
+{
+	struct iommu_map_entry *nentry;
+
+	IOMMU_DOMAIN_ASSERT_LOCKED(domain);
+	nentry = *r;
+	*r = NULL;
+	iommu_gas_rb_remove(domain, entry);
+	*nentry = *entry;
+	nentry->start = entry->end = clip;
+	iommu_gas_rb_insert(domain, entry);
+	iommu_gas_rb_insert(domain, nentry);
+	return (nentry);
+}
+
+static struct iommu_map_entry *
+iommu_gas_left_clip(struct iommu_domain *domain, struct iommu_map_entry *entry,
+    iommu_gaddr_t start, struct iommu_map_entry **r)
+{
+
+	IOMMU_DOMAIN_ASSERT_LOCKED(domain);
+	KASSERT(start <= entry->end,
+	    ("iommu_gas_left_clip entry (%#jx, %#jx) start %#jx",
+	    entry->start, entry->end, start));
+#ifdef INVARIANTS
+	if (start <= entry->start) {
+		struct iommu_map_entry *aentry;
+		aentry = RB_PREV(iommu_gas_entries_tree, &domain->rb_root,
+		    entry);
+		if (aentry != NULL) {
+			KASSERT(aentry->end <= start,
+			    ("iommu_gas_left_clip entry (%#jx, %#jx) next "
+			    "(%#jx, %#jx) start %#jx", entry->start, entry->end,
+			    aentry->start, aentry->end, start));
+		}
+	}
+#endif
+
+	return (iommu_gas_clip(domain, entry, start, r));
+}
+
+static struct iommu_map_entry *
+iommu_gas_right_clip(struct iommu_domain *domain, struct iommu_map_entry *entry,
+    iommu_gaddr_t end, struct iommu_map_entry **r)
+{
+
+	IOMMU_DOMAIN_ASSERT_LOCKED(domain);
+	KASSERT(end <= entry->end,
+	    ("iommu_gas_right_clip entry (%#jx, %#jx) trim %#jx",
+	    entry->start, entry->end, end));
+#ifdef INVARIANTS
+	if (end <= entry->start) {
+		struct iommu_map_entry *aentry;
+		aentry = RB_PREV(iommu_gas_entries_tree, &domain->rb_root,
+		    entry);
+		if (aentry != NULL) {
+			KASSERT(aentry->end <= end,
+			    ("iommu_gas_left_clip entry (%#jx, %#jx) next "
+			    "(%#jx, %#jx) start %#jx", entry->start, entry->end,
+			    aentry->start, aentry->end, end));
+		}
+	}
+#endif
+
+	return (iommu_gas_clip(domain, entry, end, r));
+}
+
+static void
+iommu_gas_remove_locked(struct iommu_domain *domain, iommu_gaddr_t start,
+    iommu_gaddr_t end, struct iommu_map_entry **r1, struct iommu_map_entry **r2)
+{
+	struct iommu_map_entry *entry, fentry, *nentry, *lentry, *rentry;
+
+	IOMMU_DOMAIN_ASSERT_LOCKED(domain);
+	MPASS(start <= end);
+	MPASS(end <= domain->last_place->end);
+
+	/*
+	 * Find an entry which contains the supplied guest's address
+	 * start, or the first entry after the start.  Since we
+	 * asserted that start is below domain end, lentry should
+	 * exist.  Then clip it if needed.
+	 */
+	fentry.start = start;
+	fentry.end = start;
+	lentry = RB_NFIND(iommu_gas_entries_tree, &domain->rb_root, &fentry);
+	MPASS(start <= lentry->start || (lentry->start <= start &&
+	    start <= lentry->end));
+	entry = start <= lentry->start ? lentry :
+	    iommu_gas_left_clip(domain, lentry, start, r1);
+
+	/*
+	 * Find an entry which contains the supplied guest's address
+	 * end, or the last entry before the end.  Clip it if needed.
+	 */
+	fentry.start = end;
+	fentry.end = end;
+	rentry = RB_NFIND(iommu_gas_entries_tree, &domain->rb_root, &fentry);
+	if (rentry->start < end) {
+		MPASS((rentry->start <= end && end <= rentry->end) ||
+		      rentry->end <= end);
+		(void)iommu_gas_right_clip(domain, rentry, end, r2);
+	}
+
+
+	for (; entry != NULL && entry->start < end; entry = nentry) {
+		KASSERT(start <= entry->start,
+		    ("iommu_gas_remove entry (%#jx, %#jx) start %#jx",
+		    entry->start, entry->end, start));
+		KASSERT(entry->end <= end,
+		    ("iommu_gas_remove entry (%#jx, %#jx) end %#jx",
+		    entry->start, entry->end, end));
+		nentry = RB_NEXT(iommu_gas_entries_tree, &domain->rb_root,
+		    entry);
+		if ((entry->flags & IOMMU_MAP_ENTRY_RMRR) == 0) {
+			iommu_gas_free_space(entry);
+			domain->ops->unmap(domain, entry->start,
+			    entry->end - entry->start, IOMMU_PGF_WAITOK);
+			if ((entry->flags & IOMMU_MAP_ENTRY_PLACE) == 0)
+				iommu_gas_free_entry(entry);
+		}
+	}
+#ifdef INVARIANTS
+	for (entry = RB_MIN(iommu_gas_entries_tree, &domain->rb_root);
+	    entry != NULL; entry = RB_NEXT(iommu_gas_entries_tree,
+	    &domain->rb_root, entry)) {
+		if ((entry->flags & IOMMU_MAP_ENTRY_RMRR) != 0)
+			continue;
+		KASSERT(entry->end <= start || entry->start >= end,
+		    ("iommu_gas_remove leftover entry (%#jx, %#jx) range "
+		    "(%#jx, %#jx)",
+		     entry->start, entry->end, start, end));
+	}
+#endif
+}
+
+void
+iommu_gas_remove(struct iommu_domain *domain, iommu_gaddr_t start,
+    iommu_gaddr_t size)
+{
+	struct iommu_map_entry *r1, *r2;
+	iommu_gaddr_t end;
+
+	end = start + size;
+	r1 = iommu_gas_alloc_entry(domain, IOMMU_PGF_WAITOK);
+	r2 = iommu_gas_alloc_entry(domain, IOMMU_PGF_WAITOK);
+	IOMMU_DOMAIN_LOCK(domain);
+	iommu_gas_remove_locked(domain, start, end, &r1, &r2);
+	IOMMU_DOMAIN_UNLOCK(domain);
+	if (r1 != NULL)
+		iommu_gas_free_entry(r1);
+	if (r2 != NULL)
+		iommu_gas_free_entry(r2);
+}
+
 int
 iommu_gas_map(struct iommu_domain *domain,
     const struct bus_dma_tag_common *common, iommu_gaddr_t size, int offset,