diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index 0728f36..a2e7921 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -123,11 +123,15 @@ And it looks like this on the wire:
 
 Flat union types avoid the nesting on the wire. They are used whenever a
 specific field of the base type is declared as the discriminator ('type' is
-then no longer generated). The discriminator must always be a string field.
+then no longer generated). The discriminator can be a string field or a
+predefined enum field. If it is a string field, a hidden enum type will be
+generated as "[UNION_NAME]Kind". If it is an enum field, a compile time check
+will be done to verify the correctness. It is recommended to use an enum field.
 The above example can then be modified as follows:
 
+ { 'enum': 'BlockdevDriver', 'data': [ 'raw', 'qcow2' ] }
  { 'type': 'BlockdevCommonOptions',
-   'data': { 'driver': 'str', 'readonly': 'bool' } }
+   'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } }
  { 'union': 'BlockdevOptions',
    'base': 'BlockdevCommonOptions',
    'discriminator': 'driver',
diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index 656a9a0..4098c60 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -201,14 +201,22 @@ def generate_union(expr):
     base = expr.get('base')
     discriminator = expr.get('discriminator')
 
+    expr_elem = {'expr': expr}
+    enum_define = discriminator_find_enum_define(expr_elem)
+    if enum_define:
+        discriminator_type_name = enum_define['enum_name']
+    else:
+        discriminator_type_name = '%sKind' % (name)
+
     ret = mcgen('''
 struct %(name)s
 {
-    %(name)sKind kind;
+    %(discriminator_type_name)s kind;
     union {
         void *data;
 ''',
-                name=name)
+                name=name,
+                discriminator_type_name=discriminator_type_name)
 
     for key in typeinfo:
         ret += mcgen('''
@@ -389,8 +397,12 @@ for expr in exprs:
         fdef.write(generate_enum_lookup(expr['enum'], expr['data']))
     elif expr.has_key('union'):
         ret += generate_fwd_struct(expr['union'], expr['data']) + "\n"
-        ret += generate_enum('%sKind' % expr['union'], expr['data'].keys())
-        fdef.write(generate_enum_lookup('%sKind' % expr['union'], expr['data'].keys()))
+        expr_elem = {'expr': expr}
+        enum_define = discriminator_find_enum_define(expr_elem)
+        if not enum_define:
+            ret += generate_enum('%sKind' % expr['union'], expr['data'].keys())
+            fdef.write(generate_enum_lookup('%sKind' % expr['union'],
+                                            expr['data'].keys()))
         if expr.get('discriminator') == {}:
             fdef.write(generate_anon_union_qtypes(expr))
     else:
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 87e6df7..08685a7 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -260,10 +260,17 @@ def generate_visit_union(expr):
         assert not base
         return generate_visit_anon_union(name, members)
 
-    # There will always be a discriminator in the C switch code, by default it
-    # is an enum type generated silently as "'%sKind' % (name)"
-    ret = generate_visit_enum('%sKind' % name, members.keys())
-    discriminator_type_name = '%sKind' % (name)
+    expr_elem = {'expr': expr}
+    enum_define = discriminator_find_enum_define(expr_elem)
+    if enum_define:
+        # Use the predefined enum type as discriminator
+        ret = ""
+        discriminator_type_name = enum_define['enum_name']
+    else:
+        # There will always be a discriminator in the C switch code, by default it
+        # is an enum type generated silently as "'%sKind' % (name)"
+        ret = generate_visit_enum('%sKind' % name, members.keys())
+        discriminator_type_name = '%sKind' % (name)
 
     if base:
         base_fields = find_struct(base)['data']
@@ -303,11 +310,12 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
     else:
         desc_type = discriminator
     ret += mcgen('''
-        visit_type_%(name)sKind(m, &(*obj)->kind, "%(type)s", &err);
+        visit_type_%(discriminator_type_name)s(m, &(*obj)->kind, "%(type)s", &err);
         if (!err) {
             switch ((*obj)->kind) {
 ''',
-                 name=name, type=desc_type)
+                 discriminator_type_name=discriminator_type_name,
+                 type=desc_type)
 
     for key in members:
         if not discriminator:
@@ -519,7 +527,12 @@ for expr in exprs:
         ret += generate_visit_list(expr['union'], expr['data'])
         fdef.write(ret)
 
-        ret = generate_decl_enum('%sKind' % expr['union'], expr['data'].keys())
+        expr_elem = {'expr': expr}
+        enum_define = discriminator_find_enum_define(expr_elem)
+        ret = ""
+        if not enum_define:
+            ret = generate_decl_enum('%sKind' % expr['union'],
+                                     expr['data'].keys())
         ret += generate_declaration(expr['union'], expr['data'])
         fdecl.write(ret)
     elif expr.has_key('enum'):
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 130dced..2a5eb59 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -250,11 +250,22 @@ def parse_schema(fp):
             add_enum(expr['enum'], expr['data'])
         elif expr.has_key('union'):
             add_union(expr)
-            add_enum('%sKind' % expr['union'])
         elif expr.has_key('type'):
             add_struct(expr)
         exprs.append(expr)
 
+    # Try again for hidden UnionKind enum
+    for expr_elem in schema.exprs:
+        expr = expr_elem['expr']
+        if expr.has_key('union'):
+            try:
+                enum_define = discriminator_find_enum_define(expr_elem)
+            except QAPIExprError, e:
+                print >>sys.stderr, e
+                exit(1)
+            if not enum_define:
+                add_enum('%sKind' % expr['union'])
+
     try:
         check_exprs(schema)
     except QAPIExprError, e:
