Index: sys/dev/iommu/iommu_gas.c
===================================================================
--- sys/dev/iommu/iommu_gas.c
+++ sys/dev/iommu/iommu_gas.c
@@ -35,6 +35,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/counter.h>
 #include <sys/malloc.h>
 #include <sys/bus.h>
 #include <sys/interrupt.h>
@@ -206,6 +207,11 @@
 }
 #endif
 
+static COUNTER_U64_DEFINE_EARLY(gas_total);
+SYSCTL_COUNTER_U64(_hw_iommu, OID_AUTO, gas_total, CTLFLAG_RD,
+    &gas_total,
+    "Sum of gas tree sizes");
+
 static void
 iommu_gas_rb_remove(struct iommu_domain *domain, struct iommu_map_entry *entry)
 {
@@ -304,6 +310,11 @@
 	struct iommu_map_entry *entry;
 };
 
+static COUNTER_U64_DEFINE_EARLY(gas_lookup);
+SYSCTL_COUNTER_U64(_hw_iommu, OID_AUTO, gas_lookup, CTLFLAG_RD,
+    &gas_lookup,
+    "Number of gas node accesses");
+
 /*
  * The interval [beg, end) is a free interval between two iommu_map_entries.
  * Addresses can be allocated only in the range [lbound, ubound). Try to
@@ -369,6 +380,7 @@
 	entry->start = start;
 	entry->end = start + roundup2(size + offset, IOMMU_PAGE_SIZE);
 	entry->flags = IOMMU_MAP_ENTRY_MAP;
+	counter_u64_add(gas_total, a->domain->entries_cnt);
 	return (true);
 }
 
@@ -381,15 +393,19 @@
 	if ((next = RB_RIGHT(curr, rb_entry)) != NULL &&
 	    next->free_down >= min_free) {
 		/* Find next entry in right subtree. */
-		do
+		do {
+			counter_u64_add(gas_lookup, 1);
 			curr = next;
-		while ((next = RB_LEFT(curr, rb_entry)) != NULL &&
+		} while ((next = RB_LEFT(curr, rb_entry)) != NULL &&
 		    next->free_down >= min_free);
 	} else {
 		/* Find next entry in a left-parent ancestor. */
 		while ((next = RB_PARENT(curr, rb_entry)) != NULL &&
-		    curr == RB_RIGHT(next, rb_entry))
+		    curr == RB_RIGHT(next, rb_entry)) {
+			counter_u64_add(gas_lookup, 1);
 			curr = next;
+		}
+		counter_u64_add(gas_lookup, 1);
 		curr = next;
 	}
 	return (curr);
@@ -421,6 +437,7 @@
 	curr = RB_ROOT(&domain->rb_root);
 	first = NULL;
 	while (curr != NULL && curr->free_down >= min_free) {
+		counter_u64_add(gas_lookup, 1);
 		first = curr;
 		curr = RB_LEFT(curr, rb_entry);
 	}
@@ -458,10 +475,13 @@
 	 * before highaddr that could abut a big-enough range.
 	 */
 	addr = a->common->highaddr;
-	while (curr != NULL && curr->last < addr)
+	while (curr != NULL && curr->last < addr) {
+		counter_u64_add(gas_lookup, 1);
 		curr = RB_PARENT(curr, rb_entry);
+	}
 	first = NULL;
 	while (curr != NULL && curr->free_down >= min_free) {
+		counter_u64_add(gas_lookup, 1);
 		if (addr < curr->end)
 			curr = RB_LEFT(curr, rb_entry);
 		else {