diff --git a/system/physmem.c b/system/physmem.c
index a7e2a5d07f..c9869e4049 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -3011,6 +3011,9 @@ static void io_mem_init(void)
 {
     memory_region_init_io(&io_mem_unassigned, NULL, &unassigned_mem_ops, NULL,
                           NULL, UINT64_MAX);
+
+    /* Trivially thread-safe since memory accesses are rejected */
+    memory_region_enable_lockless_io(&io_mem_unassigned);
 }
 
 AddressSpaceDispatch *address_space_dispatch_new(FlatView *fv)