When updating ioeventfds, we need to iterate all address spaces,
but some address spaces do not register eventfd_add|del call when
memory_listener_register() and they do nothing when updating ioeventfds.
So we can skip these AS in address_space_update_ioeventfds().

The overhead of memory_region_transaction_commit() can be significantly
reduced. For example, a VM with 8 vhost net devices and each one has
64 vectors, can reduce the time spent on memory_region_transaction_commit by 20%.

Signed-off-by: hongmianquan <hongmianquan@bytedance.com>
---
 include/exec/memory.h |  1 +
 softmmu/memory.c      | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 7f5c11a0cc..556f4f1871 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -1089,6 +1089,7 @@ struct AddressSpace {
     struct FlatView *current_map;
 
     int ioeventfd_nb;
+    int ioeventfd_notifiers;
     struct MemoryRegionIoeventfd *ioeventfds;
     QTAILQ_HEAD(, MemoryListener) listeners;
     QTAILQ_ENTRY(AddressSpace) address_spaces_link;
diff --git a/softmmu/memory.c b/softmmu/memory.c
index 7d9494ce70..77042d3f93 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -842,6 +842,10 @@ static void address_space_update_ioeventfds(AddressSpace *as)
     AddrRange tmp;
     unsigned i;
 
+    if (!as->ioeventfd_notifiers) {
+        return;
+    }
+
     /*
      * It is likely that the number of ioeventfds hasn't changed much, so use
      * the previous size as the starting value, with some headroom to avoid
@@ -3075,6 +3079,10 @@ void memory_listener_register(MemoryListener *listener, AddressSpace *as)
     }
 
     listener_add_address_space(listener, as);
+
+    if (listener->eventfd_add || listener->eventfd_del) {
+        as->ioeventfd_notifiers++;
+    }
 }
 
 void memory_listener_unregister(MemoryListener *listener)
@@ -3083,6 +3091,10 @@ void memory_listener_unregister(MemoryListener *listener)
         return;
     }
 
+    if (listener->eventfd_add || listener->eventfd_del) {
+        as->ioeventfd_notifiers--;
+    }
+
     listener_del_address_space(listener, listener->address_space);
     QTAILQ_REMOVE(&memory_listeners, listener, link);
     QTAILQ_REMOVE(&listener->address_space->listeners, listener, link_as);
-- 
2.11.0