memory: optimize empty transactions due to mutators

The mutating memory APIs can easily cause empty transactions,
where the mutators don't actually change anything, or perhaps
only modify disabled regions.  Detect these conditions and
avoid regenerating the memory topology.

Signed-off-by: Avi Kivity <avi@redhat.com>

memory.c

@@ -19,6 +19,7 @@
 #include <assert.h>
 
 unsigned memory_region_transaction_depth = 0;
+static bool memory_region_update_pending = false;
 
 typedef struct AddrRange AddrRange;
 
@@ -757,6 +758,7 @@ static void address_space_update_topology(AddressSpace *as)
 static void memory_region_update_topology(MemoryRegion *mr)
 {
     if (memory_region_transaction_depth) {
+        memory_region_update_pending |= !mr || mr->enabled;
         return;
     }
 
@@ -770,6 +772,8 @@ static void memory_region_update_topology(MemoryRegion *mr)
     if (address_space_io.root) {
         address_space_update_topology(&address_space_io);
     }
+
+    memory_region_update_pending = false;
 }
 
 void memory_region_transaction_begin(void)
@@ -781,7 +785,9 @@ void memory_region_transaction_commit(void)
 {
     assert(memory_region_transaction_depth);
     --memory_region_transaction_depth;
-    memory_region_update_topology(NULL);
+    if (!memory_region_transaction_depth && memory_region_update_pending) {
+        memory_region_update_topology(NULL);
+    }
 }
 
 static void memory_region_destructor_none(MemoryRegion *mr)