qapi: New QMP command query-qmp-schema for QMP introspection

qapi/introspect.json defines the introspection schema.  It's designed
for QMP introspection, but should do for similar uses, such as QGA.

The introspection schema does not reflect all the rules and
restrictions that apply to QAPI schemata.  A valid QAPI schema has an
introspection value conforming to the introspection schema, but the
converse is not true.

Introspection lowers away a number of schema details, and makes
implicit things explicit:

* The built-in types are declared with their JSON type.

  All integer types are mapped to 'int', because how many bits we use
  internally is an implementation detail.  It could be pressed into
  external interface service as very approximate range information,
  but that's a bad idea.  If we need range information, we better do
  it properly.

* Implicit type definitions are made explicit, and given
  auto-generated names:

  - Array types, named by appending "List" to the name of their
    element type, like in generated C.

  - The enumeration types implicitly defined by simple union types,
    named by appending "Kind" to the name of their simple union type,
    like in generated C.

  - Types that don't occur in generated C.  Their names start with ':'
    so they don't clash with the user's names.

* All type references are by name.

* The struct and union types are generalized into an object type.

* Base types are flattened.

* Commands take a single argument and return a single result.

  Dictionary argument or list result is an implicit type definition.

  The empty object type is used when a command takes no arguments or
  produces no results.

  The argument is always of object type, but the introspection schema
  doesn't reflect that.

  The 'gen': false directive is omitted as implementation detail.

  The 'success-response' directive is omitted as well for now, even
  though it's not an implementation detail, because it's not used by
  QMP.

* Events carry a single data value.

  Implicit type definition and empty object type use, just like for
  commands.

  The value is of object type, but the introspection schema doesn't
  reflect that.

* Types not used by commands or events are omitted.

  Indirect use counts as use.

* Optional members have a default, which can only be null right now

  Instead of a mandatory "optional" flag, we have an optional default.
  No default means mandatory, default null means optional without
  default value.  Non-null is available for optional with default
  (possible future extension).

* Clients should *not* look up types by name, because type names are
  not ABI.  Look up the command or event you're interested in, then
  follow the references.

  TODO Should we hide the type names to eliminate the temptation?

New generator scripts/qapi-introspect.py computes an introspection
value for its input, and generates a C variable holding it.

It can generate awfully long lines.  Marked TODO.

A new test-qmp-input-visitor test case feeds its result for both
tests/qapi-schema/qapi-schema-test.json and qapi-schema.json to a
QmpInputVisitor to verify it actually conforms to the schema.

New QMP command query-qmp-schema takes its return value from that
variable.  Its reply is some 85KiBytes for me right now.

If this turns out to be too much, we have a couple of options:

* We can use shorter names in the JSON.  Not the QMP style.

* Optionally return the sub-schema for commands and events given as
  arguments.

  Right now qmp_query_schema() sends the string literal computed by
  qmp-introspect.py.  To compute sub-schema at run time, we'd have to
  duplicate parts of qapi-introspect.py in C.  Unattractive.

* Let clients cache the output of query-qmp-schema.

  It changes only on QEMU upgrades, i.e. rarely.  Provide a command
  query-qmp-schema-hash.  Clients can have a cache indexed by hash,
  and re-query the schema only when they don't have it cached.  Even
  simpler: put the hash in the QMP greeting.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

27 files changed, 830 insertions, 11 deletions

diff --git a/.gitignore b/.gitignore
index 367bc706d8..fdd721dba1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -34,6 +34,7 @@
 /qapi-visit.[ch]
 /qapi-event.[ch]
 /qmp-commands.h
+/qmp-introspect.[ch]
 /qmp-marshal.c
 /qemu-doc.html
 /qemu-tech.html
diff --git a/Makefile b/Makefile
index 291fb54d35..8ec9b69b2f 100644
--- a/Makefile
+++ b/Makefile
@@ -52,6 +52,8 @@ endif
 GENERATED_HEADERS = config-host.h qemu-options.def
 GENERATED_HEADERS += qmp-commands.h qapi-types.h qapi-visit.h qapi-event.h
 GENERATED_SOURCES += qmp-marshal.c qapi-types.c qapi-visit.c qapi-event.c
+GENERATED_HEADERS += qmp-introspect.h
+GENERATED_SOURCES += qmp-introspect.c
 
 GENERATED_HEADERS += trace/generated-events.h
 GENERATED_SOURCES += trace/generated-events.c
@@ -269,7 +271,7 @@ $(SRC_PATH)/qga/qapi-schema.json $(SRC_PATH)/scripts/qapi-commands.py $(qapi-py)
 
 qapi-modules = $(SRC_PATH)/qapi-schema.json $(SRC_PATH)/qapi/common.json \
                $(SRC_PATH)/qapi/block.json $(SRC_PATH)/qapi/block-core.json \
-               $(SRC_PATH)/qapi/event.json
+               $(SRC_PATH)/qapi/event.json $(SRC_PATH)/qapi/introspect.json
 
 qapi-types.c qapi-types.h :\
 $(qapi-modules) $(SRC_PATH)/scripts/qapi-types.py $(qapi-py)
@@ -291,6 +293,11 @@ $(qapi-modules) $(SRC_PATH)/scripts/qapi-commands.py $(qapi-py)
 	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-commands.py \
 		$(gen-out-type) -o "." -m $<, \
 		"  GEN   $@")
+qmp-introspect.h qmp-introspect.c :\
+$(qapi-modules) $(SRC_PATH)/scripts/qapi-introspect.py $(qapi-py)
+	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-introspect.py \
+		$(gen-out-type) -o "." $<, \
+		"  GEN   $@")
 
 QGALIB_GEN=$(addprefix qga/qapi-generated/, qga-qapi-types.h qga-qapi-visit.h qga-qmp-commands.h)
 $(qga-obj-y) qemu-ga.o: $(QGALIB_GEN)
diff --git a/Makefile.objs b/Makefile.objs
index 3df2efca1c..ce87778828 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -1,7 +1,8 @@
 #######################################################################
 # Common libraries for tools and emulators
 stub-obj-y = stubs/
-util-obj-y = util/ qobject/ qapi/ qapi-types.o qapi-visit.o qapi-event.o
+util-obj-y = util/ qobject/ qapi/
+util-obj-y += qmp-introspect.o qapi-types.o qapi-visit.o qapi-event.o
 
 #######################################################################
 # block-obj-y is code used by both qemu system emulation and qemu-img
@@ -92,6 +93,7 @@ common-obj-$(CONFIG_FDT) += device_tree.o
 # qapi
 
 common-obj-y += qmp-marshal.o
+common-obj-y += qmp-introspect.o
 common-obj-y += qmp.o hmp.o
 endif
 
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index b917962315..f321d4b949 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -502,13 +502,206 @@ Resulting in this JSON object:
   "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
 
 
+== Client JSON Protocol introspection ==
+
+Clients of a Client JSON Protocol commonly need to figure out what
+exactly the server (QEMU) supports.
+
+For this purpose, QMP provides introspection via command
+query-qmp-schema.  QGA currently doesn't support introspection.
+
+query-qmp-schema returns a JSON array of SchemaInfo objects.  These
+objects together describe the wire ABI, as defined in the QAPI schema.
+
+However, the SchemaInfo can't reflect all the rules and restrictions
+that apply to QMP.  It's interface introspection (figuring out what's
+there), not interface specification.  The specification is in the QAPI
+schema.  To understand how QMP is to be used, you need to study the
+QAPI schema.
+
+Like any other command, query-qmp-schema is itself defined in the QAPI
+schema, along with the SchemaInfo type.  This text attempts to give an
+overview how things work.  For details you need to consult the QAPI
+schema.
+
+SchemaInfo objects have common members "name" and "meta-type", and
+additional variant members depending on the value of meta-type.
+
+Each SchemaInfo object describes a wire ABI entity of a certain
+meta-type: a command, event or one of several kinds of type.
+
+SchemaInfo for entities defined in the QAPI schema have the same name
+as in the schema.  This is the case for all commands and events, and
+most types.
+
+Command and event names are part of the wire ABI, but type names are
+not.  Therefore, looking up a type by its name in the QAPI schema is
+wrong.  Look up the command or event, then follow references by name.
+
+QAPI schema definitions not reachable that way are omitted.
+
+The SchemaInfo for a command has meta-type "command", and variant
+members "arg-type" and "ret-type".  On the wire, the "arguments"
+member of a client's "execute" command must conform to the object type
+named by "arg-type".  The "return" member that the server passes in a
+success response conforms to the type named by "ret-type".
+
+If the command takes no arguments, "arg-type" names an object type
+without members.  Likewise, if the command returns nothing, "ret-type"
+names an object type without members.
+
+Example: the SchemaInfo for command query-qmp-schema
+
+    { "name": "query-qmp-schema", "meta-type": "command",
+      "arg-type": ":empty", "ret-type": "SchemaInfoList" }
+
+    Type ":empty" is an object type without members, and type
+    "SchemaInfoList" is the array of SchemaInfo type.
+
+The SchemaInfo for an event has meta-type "event", and variant member
+"arg-type".  On the wire, a "data" member that the server passes in an
+event conforms to the object type named by "arg-type".
+
+If the event carries no additional information, "arg-type" names an
+object type without members.  The event may not have a data member on
+the wire then.
+
+Each command or event defined with dictionary-valued 'data' in the
+QAPI schema implicitly defines an object type called ":obj-NAME-arg",
+where NAME is the command or event's name.
+
+Example: the SchemaInfo for EVENT_C from section Events
+
+    { "name": "EVENT_C", "meta-type": "event",
+      "arg-type": ":obj-EVENT_C-arg" }
+
+    Type ":obj-EVENT_C-arg" is an implicitly defined object type with
+    the two members from the event's definition.
+
+The SchemaInfo for struct and union types has meta-type "object".
+
+The SchemaInfo for a struct type has variant member "members".
+
+The SchemaInfo for a union type additionally has variant members "tag"
+and "variants".
+
+"members" is a JSON array describing the object's common members, if
+any.  Each element is a JSON object with members "name" (the member's
+name), "type" (the name of its type), and optionally "default".  The
+member is optional if "default" is present.  Currently, "default" can
+only have value null.  Other values are reserved for future
+extensions.
+
+Example: the SchemaInfo for MyType from section Struct types
+
+    { "name": "MyType", "meta-type": "object",
+      "members": [
+          { "name": "member1", "type": "str" },
+          { "name": "member2", "type": "int" },
+          { "name": "member3", "type": "str", "default": null } ] }
+
+"tag" is the name of the common member serving as type tag.
+"variants" is a JSON array describing the object's variant members.
+Each element is a JSON object with members "case" (the value of type
+tag this element applies to) and "type" (the name of an object type
+that provides the variant members for this type tag value).
+
+Example: the SchemaInfo for flat union BlockdevOptions from section
+Union types
+
+    { "name": "BlockdevOptions", "meta-type": "object",
+      "members": [
+          { "name": "driver", "type": "BlockdevDriver" },
+          { "name": "readonly", "type": "bool"} ],
+      "tag": "driver",
+      "variants": [
+          { "case": "file", "type": "FileOptions" },
+          { "case": "qcow2", "type": "Qcow2Options" } ] }
+
+Note that base types are "flattened": its members are included in the
+"members" array.
+
+A simple union implicitly defines an enumeration type for its implicit
+discriminator (called "type" on the wire, see section Union types).
+Such a type's name is made by appending "Kind" to the simple union's
+name.
+
+A simple union implicitly defines an object type for each of its
+variants.  The type's name is ":obj-NAME-wrapper", where NAME is the
+name of the name of the variant's type.
+
+Example: the SchemaInfo for simple union BlockdevOptions from section
+Union types
+
+    { "name": "BlockdevOptions", "meta-type": "object",
+      "members": [
+          { "name": "kind", "type": "BlockdevOptionsKind" } ],
+      "tag": "type",
+      "variants": [
+          { "case": "file", "type": ":obj-FileOptions-wrapper" },
+          { "case": "qcow2", "type": ":obj-Qcow2Options-wrapper" } ] }
+
+    Enumeration type "BlockdevOptionsKind" and the object types
+    ":obj-FileOptions-wrapper", ":obj-Qcow2Options-wrapper" are
+    implicitly defined.
+
+The SchemaInfo for an alternate type has meta-type "alternate", and
+variant member "members".  "members" is a JSON array.  Each element is
+a JSON object with member "type", which names a type.  Values of the
+alternate type conform to exactly one of its member types.
+
+Example: the SchemaInfo for BlockRef from section Alternate types
+
+    { "name": "BlockRef", "meta-type": "alternate",
+      "members": [
+          { "type": "BlockdevOptions" },
+          { "type": "str" } ] }
+
+The SchemaInfo for an array type has meta-type "array", and variant
+member "element-type", which names the array's element type.  Array
+types are implicitly defined.  An array type's name is made by
+appending "List" to its element type's name.
+
+Example: the SchemaInfo for ['str']
+
+    { "name": "strList", "meta-type": "array",
+      "element-type": "str" }
+
+The SchemaInfo for an enumeration type has meta-type "enum" and
+variant member "values".
+
+Example: the SchemaInfo for MyEnum from section Enumeration types
+
+    { "name": "MyEnum", "meta-type": "enum",
+      "values": [ "value1", "value2", "value3" ] }
+
+The SchemaInfo for a built-in type has the same name as the type in
+the QAPI schema (see section Built-in Types), with one exception
+detailed below.  It has variant member "json-type" that shows how
+values of this type are encoded on the wire.
+
+Example: the SchemaInfo for str
+
+    { "name": "str", "meta-type": "builtin", "json-type": "string" }
+
+The QAPI schema supports a number of integer types that only differ in
+how they map to C.  They are identical as far as SchemaInfo is
+concerned.  Therefore, they get all mapped to a single type "int" in
+SchemaInfo.
+
+As explained above, type names are not part of the wire ABI.  Not even
+the names of built-in types.  Clients should examine member
+"json-type" instead of hard-coding names of built-in types.
+
+
 == Code generation ==
 
-Schemas are fed into 3 scripts to generate all the code/files that, paired
-with the core QAPI libraries, comprise everything required to take JSON
-commands read in by a Client JSON Protocol server, unmarshal the arguments into
-the underlying C types, call into the corresponding C function, and map the
-response back to a Client JSON Protocol response to be returned to the user.
+Schemas are fed into four scripts to generate all the code/files that,
+paired with the core QAPI libraries, comprise everything required to
+take JSON commands read in by a Client JSON Protocol server, unmarshal
+the arguments into the underlying C types, call into the corresponding
+C function, and map the response back to a Client JSON Protocol
+response to be returned to the user.
 
 As an example, we'll use the following schema, which describes a single
 complex user-defined type (which will produce a C struct, along with a list
@@ -856,3 +1049,37 @@ Example:
     extern const char *const example_QAPIEvent_lookup[];
 
     #endif
+
+=== scripts/qapi-introspect.py ===
+
+Used to generate the introspection C code for a schema. The following
+files are created:
+
+$(prefix)qmp-introspect.c - Defines a string holding a JSON
+                            description of the schema.
+$(prefix)qmp-introspect.h - Declares the above string.
+
+Example:
+
+    $ python scripts/qapi-introspect.py --output-dir="qapi-generated"
+    --prefix="example-" example-schema.json
+    $ cat qapi-generated/example-qmp-introspect.c
+[Uninteresting stuff omitted...]
+
+    const char example_qmp_schema_json[] = "["
+        "{\"arg-type\": \":empty\", \"meta-type\": \"event\", \"name\": \"MY_EVENT\"}, "
+        "{\"json-type\": \"int\", \"meta-type\": \"builtin\", \"name\": \"int\"}, "
+        "{\"json-type\": \"string\", \"meta-type\": \"builtin\", \"name\": \"str\"}, "
+        "{\"members\": [], \"meta-type\": \"object\", \"name\": \":empty\"}, "
+        "{\"members\": [{\"name\": \"arg1\", \"type\": \"UserDefOne\"}], \"meta-type\": \"object\", \"name\": \":obj-my-command-arg\"}, "
+        "{\"members\": [{\"name\": \"integer\", \"type\": \"int\"}, {\"name\": \"string\", \"type\": \"str\"}], \"meta-type\": \"object\", \"name\": \"UserDefOne\"}, "
+        "{\"arg-type\": \":obj-my-command-arg\", \"meta-type\": \"command\", \"name\": \"my-command\", \"ret-type\": \"UserDefOne\"}]";
+    $ cat qapi-generated/example-qmp-introspect.h
+[Uninteresting stuff omitted...]
+
+    #ifndef EXAMPLE_QMP_INTROSPECT_H
+    #define EXAMPLE_QMP_INTROSPECT_H
+
+    extern const char example_qmp_schema_json[];
+
+    #endif
diff --git a/monitor.c b/monitor.c
index dde1f220a1..d0edb3bb15 100644
--- a/monitor.c
+++ b/monitor.c
@@ -74,6 +74,7 @@
 #include "block/qapi.h"
 #include "qapi/qmp-event.h"
 #include "qapi-event.h"
+#include "qmp-introspect.h"
 #include "sysemu/block-backend.h"
 
 /* for hmp_info_irq/pic */
@@ -928,6 +929,21 @@ EventInfoList *qmp_query_events(Error **errp)
     return ev_list;
 }
 
+/*
+ * Minor hack: generated marshalling suppressed for this command
+ * ('gen': false in the schema) so we can parse the JSON string
+ * directly into QObject instead of first parsing it with
+ * visit_type_SchemaInfoList() into a SchemaInfoList, then marshal it
+ * to QObject with generated output marshallers, every time.  Instead,
+ * we do it in test-qmp-input-visitor.c, just to make sure
+ * qapi-introspect.py's output actually conforms to the schema.
+ */
+static void qmp_query_qmp_schema(QDict *qdict, QObject **ret_data,
+                                 Error **errp)
+{
+    *ret_data = qobject_from_json(qmp_schema_json);
+}
+
 /* set the current CPU defined by the user */
 int monitor_set_cpu(int cpu_index)
 {
diff --git a/qapi-schema.json b/qapi-schema.json
index 3ff9fec61d..821362d637 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -17,6 +17,9 @@
 # Tracing commands
 { 'include': 'qapi/trace.json' }
 
+# QAPI introspection
+{ 'include': 'qapi/introspect.json' }
+
 ##
 # @LostTickPolicy:
 #
diff --git a/qapi/introspect.json b/qapi/introspect.json
new file mode 100644
index 0000000000..9c8ad53d7a
--- /dev/null
+++ b/qapi/introspect.json
@@ -0,0 +1,277 @@
+# -*- Mode: Python -*-
+#
+# QAPI/QMP introspection
+#
+# Copyright (C) 2015 Red Hat, Inc.
+#
+# Authors:
+#  Markus Armbruster <armbru@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2 or later.
+# See the COPYING file in the top-level directory.
+
+##
+# @query-qmp-schema
+#
+# Command query-qmp-schema exposes the QMP wire ABI as an array of
+# SchemaInfo.  This lets QMP clients figure out what commands and
+# events are available in this QEMU, and their parameters and results.
+#
+# However, the SchemaInfo can't reflect all the rules and restrictions
+# that apply to QMP.  It's interface introspection (figuring out
+# what's there), not interface specification.  The specification is in
+# the QAPI schema.
+#
+# Returns: array of @SchemaInfo, where each element describes an
+# entity in the ABI: command, event, type, ...
+#
+# Note: the QAPI schema is also used to help define *internal*
+# interfaces, by defining QAPI types.  These are not part of the QMP
+# wire ABI, and therefore not returned by this command.
+#
+# Since: 2.5
+##
+{ 'command': 'query-qmp-schema',
+  'returns': [ 'SchemaInfo' ],
+  'gen': false }                # just to simplify qmp_query_json()
+
+##
+# @SchemaMetaType
+#
+# This is a @SchemaInfo's meta type, i.e. the kind of entity it
+# describes.
+#
+# @builtin: a predefined type such as 'int' or 'bool'.
+#
+# @enum: an enumeration type
+#
+# @array: an array type
+#
+# @object: an object type (struct or union)
+#
+# @alternate: an alternate type
+#
+# @command: a QMP command
+#
+# @event: a QMP event
+#
+# Since: 2.5
+##
+{ 'enum': 'SchemaMetaType',
+  'data': [ 'builtin', 'enum', 'array', 'object', 'alternate',
+            'command', 'event' ] }
+
+##
+# @SchemaInfoBase
+#
+# Members common to any @SchemaInfo.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoBase',
+  'data': { 'name': 'str', 'meta-type': 'SchemaMetaType' } }
+
+##
+# @SchemaInfo
+#
+# @name: the entity's name, inherited from @base.
+#         Entities defined in the QAPI schema have the name defined in
+#         the schema.  Implicitly defined entities have generated
+#         names.  See docs/qapi-code-gen.txt section "Client JSON
+#         Protocol introspection" for details.
+#
+# All references to other SchemaInfo are by name.
+#
+# Command and event names are part of the wire ABI, but type names are
+# not.  Therefore, looking up a type by "well-known" name is wrong.
+# Look up the command or event, then follow the references.
+#
+# @meta-type: the entity's meta type, inherited from @base.
+#
+# Additional members depend on the value of @meta-type.
+#
+# Since: 2.5
+##
+{ 'union': 'SchemaInfo',
+  'base': 'SchemaInfoBase',
+  'discriminator': 'meta-type',
+  'data': {
+      'builtin': 'SchemaInfoBuiltin',
+      'enum': 'SchemaInfoEnum',
+      'array': 'SchemaInfoArray',
+      'object': 'SchemaInfoObject',
+      'alternate': 'SchemaInfoAlternate',
+      'command': 'SchemaInfoCommand',
+      'event': 'SchemaInfoEvent' } }
+
+##
+# @SchemaInfoBuiltin
+#
+# Additional SchemaInfo members for meta-type 'builtin'.
+#
+# @json-type: the JSON type used for this type on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoBuiltin',
+  'data': { 'json-type': 'JSONType' } }
+
+##
+# @JSONType
+#
+# The four primitive and two structured types according to RFC 7159
+# section 1, plus 'int' (split off 'number'), plus the obvious top
+# type 'value'.
+#
+# Since: 2.5
+##
+{ 'enum': 'JSONType',
+  'data': [ 'string', 'number', 'int', 'boolean', 'null',
+            'object', 'array', 'value' ] }
+
+##
+# @SchemaInfoEnum
+#
+# Additional SchemaInfo members for meta-type 'enum'.
+#
+# @values: the enumeration type's values.
+#
+# Values of this type are JSON string on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoEnum',
+  'data': { 'values': ['str'] } }
+
+##
+# @SchemaInfoArray
+#
+# Additional SchemaInfo members for meta-type 'array'.
+#
+# @element-type: the array type's element type.
+#
+# Values of this type are JSON array on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoArray',
+  'data': { 'element-type': 'str' } }
+
+##
+# @SchemaInfoObject
+#
+# Additional SchemaInfo members for meta-type 'object'.
+#
+# @members: the object type's (non-variant) members.
+#
+# @tag: #optional the name of the member serving as type tag.
+#       An element of @members with this name must exist.
+#
+# @variants: #optional variant members, i.e. additional members that
+#            depend on the type tag's value.  Present exactly when
+#            @tag is present.
+#
+# Values of this type are JSON object on the wire.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObject',
+  'data': { 'members': [ 'SchemaInfoObjectMember' ],
+            '*tag': 'str',
+            '*variants': [ 'SchemaInfoObjectVariant' ] } }
+
+##
+# @SchemaInfoObjectMember
+#
+# An object member.
+#
+# @name: the member's name, as defined in the QAPI schema.
+#
+# @type: the name of the member's type.
+#
+# @default: #optional default when used as command parameter.
+#           If absent, the parameter is mandatory.
+#           If present, the value must be null.  The parameter is
+#           optional, and behavior when it's missing is not specified
+#           here.
+#           Future extension: if present and non-null, the parameter
+#           is optional, and defaults to this value.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObjectMember',
+  'data': { 'name': 'str', 'type': 'str', '*default': 'any' } }
+# @default's type must be null or match @type
+
+##
+# @SchemaInfoObjectVariant
+#
+# The variant members for a value of the type tag.
+#
+# @case: a value of the type tag.
+#
+# @type: the name of the object type that provides the variant members
+#        when the type tag has value @case.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoObjectVariant',
+  'data': { 'case': 'str', 'type': 'str' } }
+
+##
+# @SchemaInfoAlternate
+#
+# Additional SchemaInfo members for meta-type 'alternate'.
+#
+# @members: the alternate type's members.
+#           The members' wire encoding is distinct, see
+#           docs/qapi-code-gen.txt section Alternate types.
+#
+# On the wire, this can be any of the members.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoAlternate',
+  'data': { 'members': [ 'SchemaInfoAlternateMember' ] } }
+
+##
+# @SchemaInfoAlternateMember
+#
+# An alternate member.
+#
+# @type: the name of the member's type.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoAlternateMember',
+  'data': { 'type': 'str' } }
+
+##
+# @SchemaInfoCommand
+#
+# Additional SchemaInfo members for meta-type 'command'.
+#
+# @arg-type: the name of the object type that provides the command's
+#            parameters.
+#
+# @ret-type: the name of the command's result type.
+#
+# TODO @success-response (currently irrelevant, because it's QGA, not QMP)
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoCommand',
+  'data': { 'arg-type': 'str', 'ret-type': 'str' } }
+
+##
+# @SchemaInfoEvent
+#
+# Additional SchemaInfo members for meta-type 'event'.
+#
+# @arg-type: the name of the object type that provides the event's
+#            parameters.
+#
+# Since: 2.5
+##
+{ 'struct': 'SchemaInfoEvent',
+  'data': { 'arg-type': 'str' } }
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 5a5440636d..66f03007ec 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2193,6 +2193,23 @@ EQMP
     },
 
 SQMP
+query-qmp-schema
+----------------
+
+Return the QMP wire schema.  The returned value is a json-array of
+named schema entities.  Entities are commands, events and various
+types.  See docs/qapi-code-gen.txt for information on their structure
+and intended use.
+
+EQMP
+
+    {
+        .name       = "query-qmp-schema",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_query_qmp_schema,
+    },
+
+SQMP
 query-chardev
 -------------
 
diff --git a/scripts/qapi-introspect.py b/scripts/qapi-introspect.py
new file mode 100644
index 0000000000..831d6d5944
--- /dev/null
+++ b/scripts/qapi-introspect.py
@@ -0,0 +1,184 @@
+#
+# QAPI introspection generator
+#
+# Copyright (C) 2015 Red Hat, Inc.
+#
+# Authors:
+#  Markus Armbruster <armbru@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2.
+# See the COPYING file in the top-level directory.
+
+from qapi import *
+
+
+# Caveman's json.dumps() replacement (we're stuck at Python 2.4)
+# TODO try to use json.dumps() once we get unstuck
+def to_json(obj, level=0):
+    if obj is None:
+        ret = 'null'
+    elif isinstance(obj, str):
+        ret = '"' + obj.replace('"', r'\"') + '"'
+    elif isinstance(obj, list):
+        elts = [to_json(elt, level + 1)
+                for elt in obj]
+        ret = '[' + ', '.join(elts) + ']'
+    elif isinstance(obj, dict):
+        elts = ['"%s": %s' % (key.replace('"', r'\"'),
+                              to_json(obj[key], level + 1))
+                for key in sorted(obj.keys())]
+        ret = '{' + ', '.join(elts) + '}'
+    else:
+        assert False                # not implemented
+    if level == 1:
+        ret = '\n' + ret
+    return ret
+
+
+def to_c_string(string):
+    return '"' + string.replace('\\', r'\\').replace('"', r'\"') + '"'
+
+
+class QAPISchemaGenIntrospectVisitor(QAPISchemaVisitor):
+    def __init__(self):
+        self.defn = None
+        self.decl = None
+        self._schema = None
+        self._jsons = None
+        self._used_types = None
+
+    def visit_begin(self, schema):
+        self._schema = schema
+        self._jsons = []
+        self._used_types = []
+        return QAPISchemaType   # don't visit types for now
+
+    def visit_end(self):
+        # visit the types that are actually used
+        for typ in self._used_types:
+            typ.visit(self)
+        self._jsons.sort()
+        # generate C
+        # TODO can generate awfully long lines
+        name = prefix + 'qmp_schema_json'
+        self.decl = mcgen('''
+extern const char %(c_name)s[];
+''',
+                          c_name=c_name(name))
+        lines = to_json(self._jsons).split('\n')
+        c_string = '\n    '.join([to_c_string(line) for line in lines])
+        self.defn = mcgen('''
+const char %(c_name)s[] = %(c_string)s;
+''',
+                          c_name=c_name(name),
+                          c_string=c_string)
+        self._schema = None
+        self._jsons = None
+        self._used_types = None
+
+    def _use_type(self, typ):
+        # Map the various integer types to plain int
+        if typ.json_type() == 'int':
+            typ = self._schema.lookup_type('int')
+        elif (isinstance(typ, QAPISchemaArrayType) and
+              typ.element_type.json_type() == 'int'):
+            typ = self._schema.lookup_type('intList')
+        # Add type to work queue if new
+        if typ not in self._used_types:
+            self._used_types.append(typ)
+        return typ.name
+
+    def _gen_json(self, name, mtype, obj):
+        obj['name'] = name
+        obj['meta-type'] = mtype
+        self._jsons.append(obj)
+
+    def _gen_member(self, member):
+        ret = {'name': member.name, 'type': self._use_type(member.type)}
+        if member.optional:
+            ret['default'] = None
+        return ret
+
+    def _gen_variants(self, tag_name, variants):
+        return {'tag': tag_name,
+                'variants': [self._gen_variant(v) for v in variants]}
+
+    def _gen_variant(self, variant):
+        return {'case': variant.name, 'type': self._use_type(variant.type)}
+
+    def visit_builtin_type(self, name, info, json_type):
+        self._gen_json(name, 'builtin', {'json-type': json_type})
+
+    def visit_enum_type(self, name, info, values, prefix):
+        self._gen_json(name, 'enum', {'values': values})
+
+    def visit_array_type(self, name, info, element_type):
+        self._gen_json(name, 'array',
+                       {'element-type': self._use_type(element_type)})
+
+    def visit_object_type_flat(self, name, info, members, variants):
+        obj = {'members': [self._gen_member(m) for m in members]}
+        if variants:
+            obj.update(self._gen_variants(variants.tag_member.name,
+                                          variants.variants))
+        self._gen_json(name, 'object', obj)
+
+    def visit_alternate_type(self, name, info, variants):
+        self._gen_json(name, 'alternate',
+                       {'members': [{'type': self._use_type(m.type)}
+                                    for m in variants.variants]})
+
+    def visit_command(self, name, info, arg_type, ret_type,
+                      gen, success_response):
+        arg_type = arg_type or self._schema.the_empty_object_type
+        ret_type = ret_type or self._schema.the_empty_object_type
+        self._gen_json(name, 'command',
+                       {'arg-type': self._use_type(arg_type),
+                        'ret-type': self._use_type(ret_type)})
+
+    def visit_event(self, name, info, arg_type):
+        arg_type = arg_type or self._schema.the_empty_object_type
+        self._gen_json(name, 'event', {'arg-type': self._use_type(arg_type)})
+
+(input_file, output_dir, do_c, do_h, prefix, dummy) = parse_command_line()
+
+c_comment = '''
+/*
+ * QAPI/QMP schema introspection
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+'''
+h_comment = '''
+/*
+ * QAPI/QMP schema introspection
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+'''
+
+(fdef, fdecl) = open_output(output_dir, do_c, do_h, prefix,
+                            'qmp-introspect.c', 'qmp-introspect.h',
+                            c_comment, h_comment)
+
+fdef.write(mcgen('''
+#include "%(prefix)sqmp-introspect.h"
+
+''',
+                 prefix=prefix))
+
+schema = QAPISchema(input_file)
+gen = QAPISchemaGenIntrospectVisitor()
+schema.visit(gen)
+fdef.write(gen.defn)
+fdecl.write(gen.decl)
+
+close_output(fdef, fdecl)
diff --git a/scripts/qapi.py b/scripts/qapi.py
index f4072978c1..06478bb269 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -788,6 +788,9 @@ class QAPISchemaVisitor(object):
     def visit_object_type(self, name, info, base, members, variants):
         pass
 
+    def visit_object_type_flat(self, name, info, members, variants):
+        pass
+
     def visit_alternate_type(self, name, info, variants):
         pass
 
@@ -943,6 +946,8 @@ class QAPISchemaObjectType(QAPISchemaType):
     def visit(self, visitor):
         visitor.visit_object_type(self.name, self.info,
                                   self.base, self.local_members, self.variants)
+        visitor.visit_object_type_flat(self.name, self.info,
+                                       self.members, self.variants)
 
 
 class QAPISchemaObjectTypeMember(object):
@@ -1113,6 +1118,9 @@ class QAPISchema(object):
                   ('bool',   'boolean', 'bool',     'false'),
                   ('any',    'value',   'QObject' + pointer_suffix, 'NULL')]:
             self._def_builtin_type(*t)
+        self.the_empty_object_type = QAPISchemaObjectType(':empty', None, None,
+                                                          [], None)
+        self._def_entity(self.the_empty_object_type)
 
     def _make_implicit_enum_type(self, name, values):
         name = name + 'Kind'
@@ -1260,9 +1268,10 @@ class QAPISchema(object):
             ent.check(self)
 
     def visit(self, visitor):
-        visitor.visit_begin(self)
+        ignore = visitor.visit_begin(self)
         for name in sorted(self._entity_dict.keys()):
-            self._entity_dict[name].visit(visitor)
+            if not ignore or not isinstance(self._entity_dict[name], ignore):
+                self._entity_dict[name].visit(visitor)
         visitor.visit_end()
 
 
diff --git a/tests/.gitignore b/tests/.gitignore
index 2c5e2c31e4..a607bddce4 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -35,6 +35,7 @@ test-qmp-commands.h
 test-qmp-event
 test-qmp-input-strict
 test-qmp-input-visitor
+test-qmp-introspect.[ch]
 test-qmp-marshal.c
 test-qmp-output-visitor
 test-rcu-list
diff --git a/tests/Makefile b/tests/Makefile
index aadfb38fdc..4063639a59 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -269,7 +269,8 @@ check-qapi-schema-y := $(addprefix tests/qapi-schema/, \
 	struct-base-clash.json struct-base-clash-deep.json )
 
 GENERATED_HEADERS += tests/test-qapi-types.h tests/test-qapi-visit.h \
-		     tests/test-qmp-commands.h tests/test-qapi-event.h
+	tests/test-qmp-commands.h tests/test-qapi-event.h \
+	tests/test-qmp-introspect.h
 
 test-obj-y = tests/check-qint.o tests/check-qstring.o tests/check-qdict.o \
 	tests/check-qlist.o tests/check-qfloat.o tests/check-qjson.o \
@@ -289,7 +290,7 @@ QEMU_CFLAGS += -I$(SRC_PATH)/tests
 test-util-obj-y = libqemuutil.a libqemustub.a
 test-qom-obj-y = $(qom-obj-y) $(test-util-obj-y)
 test-qapi-obj-y = tests/test-qapi-visit.o tests/test-qapi-types.o \
-	tests/test-qapi-event.o \
+	tests/test-qapi-event.o tests/test-qmp-introspect.o \
 	$(test-qom-obj-y)
 test-crypto-obj-y = $(crypto-obj-y) $(test-qom-obj-y)
 test-block-obj-y = $(block-obj-y) $(test-crypto-obj-y)
@@ -346,6 +347,11 @@ $(SRC_PATH)/tests/qapi-schema/qapi-schema-test.json $(SRC_PATH)/scripts/qapi-eve
 	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-event.py \
 		$(gen-out-type) -o tests -p "test-" $<, \
 		"  GEN   $@")
+tests/test-qmp-introspect.c tests/test-qmp-introspect.h :\
+$(SRC_PATH)/tests/qapi-schema/qapi-schema-test.json $(SRC_PATH)/scripts/qapi-introspect.py $(qapi-py)
+	$(call quiet-command,$(PYTHON) $(SRC_PATH)/scripts/qapi-introspect.py \
+		$(gen-out-type) -o tests -p "test-" $<, \
+		"  GEN   $@")
 
 tests/test-string-output-visitor$(EXESUF): tests/test-string-output-visitor.o $(test-qapi-obj-y)
 tests/test-string-input-visitor$(EXESUF): tests/test-string-input-visitor.o $(test-qapi-obj-y)
diff --git a/tests/qapi-schema/alternate-good.out b/tests/qapi-schema/alternate-good.out
index 3d765ffd7b..65af7278f4 100644
--- a/tests/qapi-schema/alternate-good.out
+++ b/tests/qapi-schema/alternate-good.out
@@ -1,3 +1,4 @@
+object :empty
 alternate Alt
     case value: int
     case string: Enum
diff --git a/tests/qapi-schema/args-member-array.out b/tests/qapi-schema/args-member-array.out
index b67384cc2e..b3b92dfc8e 100644
--- a/tests/qapi-schema/args-member-array.out
+++ b/tests/qapi-schema/args-member-array.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-okay-arg
     member member1: intList optional=False
     member member2: defList optional=False
diff --git a/tests/qapi-schema/comments.out b/tests/qapi-schema/comments.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/comments.out
+++ b/tests/qapi-schema/comments.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/empty.out b/tests/qapi-schema/empty.out
index e69de29bb2..272b1616f4 100644
--- a/tests/qapi-schema/empty.out
+++ b/tests/qapi-schema/empty.out
@@ -0,0 +1 @@
+object :empty
diff --git a/tests/qapi-schema/enum-empty.out b/tests/qapi-schema/enum-empty.out
index e09b00ffd6..a449d455fb 100644
--- a/tests/qapi-schema/enum-empty.out
+++ b/tests/qapi-schema/enum-empty.out
@@ -1 +1,2 @@
+object :empty
 enum MyEnum []
diff --git a/tests/qapi-schema/event-case.out b/tests/qapi-schema/event-case.out
index b5ae4c20b3..cdfd264f9c 100644
--- a/tests/qapi-schema/event-case.out
+++ b/tests/qapi-schema/event-case.out
@@ -1 +1,2 @@
+object :empty
 event oops None
diff --git a/tests/qapi-schema/flat-union-reverse-define.out b/tests/qapi-schema/flat-union-reverse-define.out
index 477fb31198..a5a9134e7e 100644
--- a/tests/qapi-schema/flat-union-reverse-define.out
+++ b/tests/qapi-schema/flat-union-reverse-define.out
@@ -1,3 +1,4 @@
+object :empty
 object TestBase
     member enum1: TestEnum optional=False
 enum TestEnum ['value1', 'value2']
diff --git a/tests/qapi-schema/ident-with-escape.out b/tests/qapi-schema/ident-with-escape.out
index 9577d1bde7..f4542b12d8 100644
--- a/tests/qapi-schema/ident-with-escape.out
+++ b/tests/qapi-schema/ident-with-escape.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-fooA-arg
     member bar1: str optional=False
 command fooA :obj-fooA-arg -> None
diff --git a/tests/qapi-schema/include-relpath.out b/tests/qapi-schema/include-relpath.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-relpath.out
+++ b/tests/qapi-schema/include-relpath.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/include-repetition.out b/tests/qapi-schema/include-repetition.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-repetition.out
+++ b/tests/qapi-schema/include-repetition.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/include-simple.out b/tests/qapi-schema/include-simple.out
index 6161b90e91..9e2c656fa0 100644
--- a/tests/qapi-schema/include-simple.out
+++ b/tests/qapi-schema/include-simple.out
@@ -1 +1,2 @@
+object :empty
 enum Status ['good', 'bad', 'ugly']
diff --git a/tests/qapi-schema/indented-expr.out b/tests/qapi-schema/indented-expr.out
index c5af55a05c..226d300798 100644
--- a/tests/qapi-schema/indented-expr.out
+++ b/tests/qapi-schema/indented-expr.out
@@ -1,3 +1,4 @@
+object :empty
 command eins None -> None
    gen=True success_response=True
 command zwei None -> None
diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
index a52ac31ea5..1f6e858def 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -1,3 +1,4 @@
+object :empty
 object :obj-EVENT_C-arg
     member a: int optional=True
     member b: UserDefOne optional=True
diff --git a/tests/qapi-schema/returns-int.out b/tests/qapi-schema/returns-int.out
index 1ac3e1e267..a2da259be4 100644
--- a/tests/qapi-schema/returns-int.out
+++ b/tests/qapi-schema/returns-int.out
@@ -1,2 +1,3 @@
+object :empty
 command guest-get-time None -> int
    gen=True success_response=True
diff --git a/tests/test-qmp-input-strict.c b/tests/test-qmp-input-strict.c
index a2ae78661b..53a769388c 100644
--- a/tests/test-qmp-input-strict.c
+++ b/tests/test-qmp-input-strict.c
@@ -19,6 +19,9 @@
 #include "test-qapi-types.h"
 #include "test-qapi-visit.h"
 #include "qapi/qmp/types.h"
+#include "test-qmp-introspect.h"
+#include "qmp-introspect.h"
+#include "qapi-visit.h"
 
 typedef struct TestInputVisitorData {
     QObject *obj;
@@ -62,6 +65,30 @@ Visitor *validate_test_init(TestInputVisitorData *data,
     return v;
 }
 
+/* similar to validate_test_init(), but does not expect a string
+ * literal/format json_string argument and so can be used for
+ * programatically generated strings (and we can't pass in programatically
+ * generated strings via %s format parameters since qobject_from_jsonv()
+ * will wrap those in double-quotes and treat the entire object as a
+ * string)
+ */
+static Visitor *validate_test_init_raw(TestInputVisitorData *data,
+                                       const char *json_string)
+{
+    Visitor *v;
+
+    data->obj = qobject_from_json(json_string);
+    g_assert(data->obj != NULL);
+
+    data->qiv = qmp_input_visitor_new_strict(data->obj);
+    g_assert(data->qiv != NULL);
+
+    v = qmp_input_get_visitor(data->qiv);
+    g_assert(v != NULL);
+
+    return v;
+}
+
 typedef struct TestStruct
 {
     int64_t integer;
@@ -293,6 +320,32 @@ static void test_validate_fail_alternate(TestInputVisitorData *data,
     qapi_free_UserDefAlternate(tmp);
 }
 
+static void do_test_validate_qmp_introspect(TestInputVisitorData *data,
+                                            const char *schema_json)
+{
+    SchemaInfoList *schema = NULL;
+    Error *err = NULL;
+    Visitor *v;
+
+    v = validate_test_init_raw(data, schema_json);
+
+    visit_type_SchemaInfoList(v, &schema, NULL, &err);
+    if (err) {
+        fprintf(stderr, "%s", error_get_pretty(err));
+    }
+    g_assert(!err);
+    g_assert(schema);
+
+    qapi_free_SchemaInfoList(schema);
+}
+
+static void test_validate_qmp_introspect(TestInputVisitorData *data,
+                                           const void *unused)
+{
+    do_test_validate_qmp_introspect(data, test_qmp_schema_json);
+    do_test_validate_qmp_introspect(data, qmp_schema_json);
+}
+
 static void validate_test_add(const char *testpath,
                                TestInputVisitorData *data,
                                void (*test_func)(TestInputVisitorData *data, const void *user_data))
@@ -333,6 +386,8 @@ int main(int argc, char **argv)
                       &testdata, test_validate_fail_alternate);
     validate_test_add("/visitor/input-strict/fail/union-native-list",
                       &testdata, test_validate_fail_union_native_list);
+    validate_test_add("/visitor/input-strict/pass/qmp-introspect",
+                      &testdata, test_validate_qmp_introspect);
 
     g_test_run();