= How to use the QAPI code generator = Copyright IBM Corp. 2011 Copyright (C) 2012-2016 Red Hat, Inc. This work is licensed under the terms of the GNU GPL, version 2 or later. See the COPYING file in the top-level directory. == Introduction == QAPI is a native C API within QEMU which provides management-level functionality to internal and external users. For external users/processes, this interface is made available by a JSON-based wire format for the QEMU Monitor Protocol (QMP) for controlling qemu, as well as the QEMU Guest Agent (QGA) for communicating with the guest. The remainder of this document uses "Client JSON Protocol" when referring to the wire contents of a QMP or QGA connection. To map Client JSON Protocol interfaces to the native C QAPI implementations, a JSON-based schema is used to define types and function signatures, and a set of scripts is used to generate types, signatures, and marshaling/dispatch code. This document will describe how the schemas, scripts, and resulting code are used. == QMP/Guest agent schema == A QAPI schema file is designed to be loosely based on JSON (http://www.ietf.org/rfc/rfc8259.txt) with changes for quoting style and the use of comments; a QAPI schema file is then parsed by a python code generation program. A valid QAPI schema consists of a series of top-level expressions, with no commas between them. Where dictionaries (JSON objects) are used, they are parsed as python OrderedDicts so that ordering is preserved (for predictable layout of generated C structs and parameter lists). Ordering doesn't matter between top-level expressions or the keys within an expression, but does matter within dictionary values for 'data' and 'returns' members of a single expression. QAPI schema input is written using 'single quotes' instead of JSON's "double quotes" (in contrast, Client JSON Protocol uses no comments, and while input accepts 'single quotes' as an extension, output is strict JSON using only "double quotes"). As in JSON, trailing commas are not permitted in arrays or dictionaries. Input must be ASCII (although QMP supports full Unicode strings, the QAPI parser does not). At present, there is no place where a QAPI schema requires the use of JSON numbers or null. === Comments === Comments are allowed; anything between an unquoted # and the following newline is ignored. === Schema overview === The schema sets up a series of types, as well as commands and events that will use those types. Forward references are allowed: the parser scans in two passes, where the first pass learns all type names, and the second validates the schema and generates the code. This allows the definition of complex structs that can have mutually recursive types, and allows for indefinite nesting of Client JSON Protocol that satisfies the schema. A type name should not be defined more than once. It is permissible for the schema to contain additional types not used by any commands or events in the Client JSON Protocol, for the side effect of generated C code used internally. There are eight top-level expressions recognized by the parser: 'include', 'pragma', 'command', 'struct', 'enum', 'union', 'alternate', and 'event'. There are several groups of types: simple types (a number of built-in types, such as 'int' and 'str'; as well as enumerations), complex types (structs and two flavors of unions), and alternate types (a choice between other types). The 'command' and 'event' expressions can refer to existing types by name, or list an anonymous type as a dictionary. Listing a type name inside an array refers to a single-dimension array of that type; multi-dimension arrays are not directly supported (although an array of a complex struct that contains an array member is possible). In the rest of this document, usage lines are given for each expression type, with literal strings written in lower case and placeholders written in capitals. If a literal string includes a prefix of '*', that key/value pair can be omitted from the expression. For example, a usage statement that includes '*base':STRUCT-NAME means that an expression has an optional key 'base', which if present must have a value that forms a struct name. === Built-in Types === The following types are predefined, and map to C as follows: Schema C JSON str char * any JSON string, UTF-8 number double any JSON number int int64_t a JSON number without fractional part that fits into the C integer type int8 int8_t likewise int16 int16_t likewise int32 int32_t likewise int64 int64_t likewise uint8 uint8_t likewise uint16 uint16_t likewise uint32 uint32_t likewise uint64 uint64_t likewise size uint64_t like uint64_t, except StringInputVisitor accepts size suffixes bool bool JSON true or false null QNull * JSON null any QObject * any JSON value QType QType JSON string matching enum QType values === Include directives === Usage: { 'include': STRING } The QAPI schema definitions can be modularized using the 'include' directive: { 'include': 'path/to/file.json' } The directive is evaluated recursively, and include paths are relative to the file using the directive. Multiple includes of the same file are idempotent. No other keys should appear in the expression, and the include value should be a string. As a matter of style, it is a good idea to have all files be self-contained, but at the moment, nothing prevents an included file from making a forward reference to a type that is only introduced by an outer file. The parser may be made stricter in the future to prevent incomplete include files. === Pragma directives === Usage: { 'pragma': DICT } The pragma directive lets you control optional generator behavior. The dictionary's entries are pragma names and values. Pragma's scope is currently the complete schema. Setting the same pragma to different values in parts of the schema doesn't work. Pragma 'doc-required' takes a boolean value. If true, documentation is required. Default is false. Pragma 'returns-whitelist' takes a list of command names that may violate the rules on permitted return types. Default is none. Pragma 'name-case-whitelist' takes a list of names that may violate rules on use of upper- vs. lower-case letters. Default is none. === Enumeration types === Usage: { 'enum': STRING, 'data': ARRAY-OF-STRING } { 'enum': STRING, '*prefix': STRING, 'data': ARRAY-OF-STRING } An enumeration type is a dictionary containing a single 'data' key whose value is a list of strings. An example enumeration is: { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] } Nothing prevents an empty enumeration, although it is probably not useful. The list of strings should be lower case; if an enum name represents multiple words, use '-' between words. The string 'max' is not allowed as an enum value, and values should not be repeated. The enum constants will be named by using a heuristic to turn the type name into a set of underscore separated words. For the example above, 'MyEnum' will turn into 'MY_ENUM' giving a constant name of 'MY_ENUM_VALUE1' for the first value. If the default heuristic does not result in a desirable name, the optional 'prefix' member can be used when defining the enum. The enumeration values are passed as strings over the Client JSON Protocol, but are encoded as C enum integral values in generated code. While the C code starts numbering at 0, it is better to use explicit comparisons to enum values than implicit comparisons to 0; the C code will also include a generated enum member ending in _MAX for tracking the size of the enum, useful when using common functions for converting between strings and enum values. Since the wire format always passes by name, it is acceptable to reorder or add new enumeration members in any location without breaking clients of Client JSON Protocol; however, removing enum values would break compatibility. For any struct that has a member that will only contain a finite set of string values, using an enum type for that member is better than open-coding the member to be type 'str'. === Struct types === Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME } A struct is a dictionary containing a single 'data' key whose value is a dictionary; the dictionary may be empty. This corresponds to a struct in C or an Object in JSON. Each value of the 'data' dictionary must be the name of a type, or a one-element array containing a type name. An example of a struct is: { 'struct': 'MyType', 'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } } The use of '*' as a prefix to the name means the member is optional in the corresponding JSON protocol usage. The default initialization value of an optional argument should not be changed between versions of QEMU unless the new default maintains backward compatibility to the user-visible behavior of the old default. With proper documentation, this policy still allows some flexibility; for example, documenting that a default of 0 picks an optimal buffer size allows one release to declare the optimal size at 512 while another release declares the optimal size at 4096 - the user-visible behavior is not the bytes used by the buffer, but the fact that the buffer was optimal size. On input structures (only mentioned in the 'data' side of a command), changing from mandatory to optional is safe (older clients will supply the option, and newer clients can benefit from the default); changing from optional to mandatory is backwards incompatible (older clients may be omitting the option, and must continue to work). On output structures (only mentioned in the 'returns' side of a command), changing from mandatory to optional is in general unsafe (older clients may be expecting the member, and could crash if it is missing), although it can be done if the only way that the optional argument will be omitted is when it is triggered by the presence of a new input flag to the command that older clients don't know to send. Changing from optional to mandatory is safe. A structure that is used in both input and output of various commands must consider the backwards compatibility constraints of both directions of use. A struct definition can specify another struct as its base. In this case, the members of the base type are included as top-level members of the new struct's dictionary in the Client JSON Protocol wire format. An example definition is: { 'struct': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } } { 'struct': 'BlockdevOptionsGenericCOWFormat', 'base': 'BlockdevOptionsGenericFormat', 'data': { '*backing': 'str' } } An example BlockdevOptionsGenericCOWFormat object on the wire could use both members like this: { "file": "/some/place/my-image", "backing": "/some/place/my-backing-file" } === Union types === Usage: { 'union': STRING, 'data': DICT } or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT, 'discriminator': ENUM-MEMBER-OF-BASE } Union types are used to let the user choose between several different variants for an object. There are two flavors: simple (no discriminator or base), and flat (both discriminator and base). A union type is defined using a data dictionary as explained in the following paragraphs. Unions must have at least one branch. A simple union type defines a mapping from automatic discriminator values to data types like in this example: { 'struct': 'BlockdevOptionsFile', 'data': { 'filename': 'str' } } { 'struct': 'BlockdevOptionsQcow2', 'data': { 'backing': 'str', '*lazy-refcounts': 'bool' } } { 'union': 'BlockdevOptionsSimple', 'data': { 'file': 'BlockdevOptionsFile', 'qcow2': 'BlockdevOptionsQcow2' } } In the Client JSON Protocol, a simple union is represented by a dictionary that contains the 'type' member as a discriminator, and a 'data' member that is of the specified data type corresponding to the discriminator value, as in these examples: { "type": "file", "data": { "filename": "/some/place/my-image" } } { "type": "qcow2", "data": { "backing": "/some/place/my-image", "lazy-refcounts": true } } The generated C code uses a struct containing a union. Additionally, an implicit C enum 'NameKind' is created, corresponding to the union 'Name', for accessing the various branches of the union. The value for each branch can be of any type. A flat union definition avoids nesting on the wire, and specifies a set of common members that occur in all variants of the union. The 'base' key must specify either a type name (the type must be a struct, not a union), or a dictionary representing an anonymous type. All branches of the union must be struct types, and the top-level members of the union dictionary on the wire will be combination of members from both the base type and the appropriate branch type (when merging two dictionaries, there must be no keys in common). The 'discriminator' member must be the name of a non-optional enum-typed member of the base struct. The following example enhances the above simple union example by adding an optional common member 'read-only', renaming the discriminator to something more applicable than the simple union's default of 'type', and reducing the number of {} required on the wire: { 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] } { 'union': 'BlockdevOptions', 'base': { 'driver': 'BlockdevDriver', '*read-only': 'bool' }, 'discriminator': 'driver', 'data': { 'file': 'BlockdevOptionsFile', 'qcow2': 'BlockdevOptionsQcow2' } } Resulting in these JSON objects: { "driver": "file", "read-only": true, "filename": "/some/place/my-image" } { "driver": "qcow2", "read-only": false, "backing": "/some/place/my-image", "lazy-refcounts": true } Notice that in a flat union, the discriminator name is controlled by the user, but because it must map to a base member with enum type, the code generator ensures that branches match the existing values of the enum. The order of the keys need not match the declaration of the enum. The keys need not cover all possible enum values. Omitted enum values are still valid branches that add no additional members to the data type. In the resulting generated C data types, a flat union is represented as a struct with the base members included directly, and then a union of structures for each branch of the struct. A simple union can always be re-written as a flat union where the base class has a single member named 'type', and where each branch of the union has a struct with a single member named 'data'. That is, { 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } } is identical on the wire to: { 'enum': 'Enum', 'data': ['one', 'two'] } { 'struct': 'Branch1', 'data': { 'data': 'str' } } { 'struct': 'Branch2', 'data': { 'data': 'int' } } { 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type', 'data': { 'one': 'Branch1', 'two': 'Branch2' } } === Alternate types === Usage: { 'alternate': STRING, 'data': DICT } An alternate type is one that allows a choice between two or more JSON data types (string, integer, number, or object, but currently not array) on the wire. The definition is similar to a simple union type, where each branch of the union names a QAPI type. For example: { 'alternate': 'BlockdevRef', 'data': { 'definition': 'BlockdevOptions', 'reference': 'str' } } Unlike a union, the discriminator string is never passed on the wire for the Client JSON Protocol. Instead, the value's JSON type serves as an implicit discriminator, which in turn means that an alternate can only express a choice between types represented differently in JSON. If a branch is typed as the 'bool' built-in, the alternate accepts true and false; if it is typed as any of the various numeric built-ins, it accepts a JSON number; if it is typed as a 'str' built-in or named enum type, it accepts a JSON string; if it is typed as the 'null' built-in, it accepts JSON null; and if it is typed as a complex type (struct or union), it accepts a JSON object. Two different complex types, for instance, aren't permitted, because both are represented as a JSON object. The example alternate declaration above allows using both of the following example objects: { "file": "my_existing_block_device_id" } { "file": { "driver": "file", "read-only": false, "filename": "/tmp/mydisk.qcow2" } } === Commands === --- General Command Layout --- Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, '*returns': TYPE-NAME, '*boxed': true, '*gen': false, '*success-response': false, '*allow-oob': true, '*allow-preconfig': true } Commands are defined by using a dictionary containing several members, where three members are most common. The 'command' member is a mandatory string, and determines the "execute" value passed in a Client JSON Protocol command exchange. The 'data' argument maps to the "arguments" dictionary passed in as part of a Client JSON Protocol command. The 'data' member is optional and defaults to {} (an empty dictionary). If present, it must be the string name of a complex type, or a dictionary that declares an anonymous type with the same semantics as a 'struct' expression. The 'returns' member describes what will appear in the "return" member of a Client JSON Protocol reply on successful completion of a command. The member is optional from the command declaration; if absent, the "return" member will be an empty dictionary. If 'returns' is present, it must be the string name of a complex type, or a one-element array containing the name of a complex type. To return anything else, you have to list the command in pragma 'returns-whitelist'. If you do this, the command cannot be extended to return additional information in the future. Use of 'returns-whitelist' for new commands is strongly discouraged. All commands in Client JSON Protocol use a dictionary to report failure, with no way to specify that in QAPI. Where the error return is different than the usual GenericError class in order to help the client react differently to certain error conditions, it is worth documenting this in the comments before the command declaration. Some example commands: { 'command': 'my-first-command', 'data': { 'arg1': 'str', '*arg2': 'str' } } { 'struct': 'MyType', 'data': { '*value': 'str' } } { 'command': 'my-second-command', 'returns': [ 'MyType' ] } which would validate this Client JSON Protocol transaction: => { "execute": "my-first-command", "arguments": { "arg1": "hello" } } <= { "return": { } } => { "execute": "my-second-command" } <= { "return": [ { "value": "one" }, { } ] } The generator emits a prototype for the user's function implementing the command. Normally, 'data' is a dictionary for an anonymous type, or names a struct type (possibly empty, but not a union), and its members are passed as separate arguments to this function. If the command definition includes a key 'boxed' with the boolean value true, then 'data' is instead the name of any non-empty complex type (struct or union), and a pointer to that QAPI type is passed as a single argument. The generator also emits a marshalling function that extracts arguments for the user's function out of an input QDict, calls the user's function, and if it succeeded, builds an output QObject from its return value. In rare cases, QAPI cannot express a type-safe representation of a corresponding Client JSON Protocol command. You then have to suppress generation of a marshalling function by including a key 'gen' with boolean value false, and instead write your own function. For example: { 'command': 'netdev_add', 'data': {'type': 'str', 'id': 'str'}, 'gen': false } Please try to avoid adding new commands that rely on this, and instead use type-safe unions. Normally, the QAPI schema is used to describe synchronous exchanges, where a response is expected. But in some cases, the action of a command is expected to change state in a way that a successful response is not possible (although the command will still return a normal dictionary error on failure). When a successful reply is not possible, the command expression includes the optional key 'success-response' with boolean value false. So far, only QGA makes use of this member. Key 'allow-oob' declares whether the command supports out-of-band (OOB) execution. It defaults to false. For example: { 'command': 'migrate_recover', 'data': { 'uri': 'str' }, 'allow-oob': true } See qmp-spec.txt for out-of-band execution syntax and semantics. Commands supporting out-of-band execution can still be executed in-band. When a command is executed in-band, its handler runs in the main thread with the BQL held. When a command is executed out-of-band, its handler runs in a dedicated monitor I/O thread with the BQL *not* held. An OOB-capable command handler must satisfy the following conditions: - It terminates quickly. - It does not invoke system calls that may block. - It does not access guest RAM that may block when userfaultfd is enabled for postcopy live migration. - It takes only "fast" locks, i.e. all critical sections protected by any lock it takes also satisfy the conditions for OOB command handler code. The restrictions on locking limit access to shared state. Such access requires synchronization, but OOB commands can't take the BQL or any other "slow" lock. When in doubt, do not implement OOB execution support. Key 'allow-preconfig' declares whether the command is available before the machine is built. It defaults to false. For example: { 'command': 'qmp_capabilities', 'data': { '*enable': [ 'QMPCapability' ] }, 'allow-preconfig': true } QMP is available before the machine is built only when QEMU was started with --preconfig. === Events === Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, '*boxed': true } Events are defined with the keyword 'event'. When 'data' is also specified, additional info will be included in the event, with similar semantics to a 'struct' expression. Finally there will be C API generated in qapi-events.h; when called by QEMU code, a message with timestamp will be emitted on the wire. An example event is: { 'event': 'EVENT_C', 'data': { '*a': 'int', 'b': 'str' } } Resulting in this JSON object: { "event": "EVENT_C", "data": { "b": "test string" }, "timestamp": { "seconds": 1267020223, "microseconds": 435656 } } The generator emits a function to send the event. Normally, 'data' is a dictionary for an anonymous type, or names a struct type (possibly empty, but not a union), and its members are passed as separate arguments to this function. If the event definition includes a key 'boxed' with the boolean value true, then 'data' is instead the name of any non-empty complex type (struct or union), and a pointer to that QAPI type is passed as a single argument. === Features === Sometimes, the behaviour of QEMU changes compatibly, but without a change in the QMP syntax (usually by allowing values or operations that previously resulted in an error). QMP clients may still need to know whether the extension is available. For this purpose, a list of features can be specified for a struct type. This is exposed to the client as a list of string, where each string signals that this build of QEMU shows a certain behaviour. In the schema, features can be specified as simple strings, for example: { 'struct': 'TestType', 'data': { 'number': 'int' }, 'features': [ 'allow-negative-numbers' ] } Another option is to specify features as dictionaries, where the key 'name' specifies the feature string to be exposed to clients: { 'struct': 'TestType', 'data': { 'number': 'int' }, 'features': [ { 'name': 'allow-negative-numbers' } ] } This expanded form is necessary if you want to make the feature conditional (see below in "Configuring the schema"). === Naming rules and reserved names === All names must begin with a letter, and contain only ASCII letters, digits, hyphen, and underscore. There are two exceptions: enum values may start with a digit, and names that are downstream extensions (see section Downstream extensions) start with underscore. Names beginning with 'q_' are reserved for the generator, which uses them for munging QMP names that resemble C keywords or other problematic strings. For example, a member named "default" in qapi becomes "q_default" in the generated C code. Types, commands, and events share a common namespace. Therefore, generally speaking, type definitions should always use CamelCase for user-defined type names, while built-in types are lowercase. Type names ending with 'Kind' or 'List' are reserved for the generator, which uses them for implicit union enums and array types, respectively. Command names, and member names within a type, should be all lower case with words separated by a hyphen. However, some existing older commands and complex types use underscore; when extending such expressions, consistency is preferred over blindly avoiding underscore. Event names should be ALL_CAPS with words separated by underscore. Member name 'u' and names starting with 'has-' or 'has_' are reserved for the generator, which uses them for unions and for tracking optional members. Any name (command, event, type, member, or enum value) beginning with "x-" is marked experimental, and may be withdrawn or changed incompatibly in a future release. Pragma 'name-case-whitelist' lets you violate the rules on use of upper and lower case. Use for new code is strongly discouraged. === Downstream extensions === QAPI schema names that are externally visible, say in the Client JSON Protocol, need to be managed with care. Names starting with a downstream prefix of the form __RFQDN_ are reserved for the downstream who controls the valid, reverse fully qualified domain name RFQDN. RFQDN may only contain ASCII letters, digits, hyphen and period. Example: Red Hat, Inc. controls redhat.com, and may therefore add a downstream command __com.redhat_drive-mirror. === Configuring the schema === The 'struct', 'enum', 'union', 'alternate', 'command' and 'event' top-level expressions can take an 'if' key. Its value must be a string or a list of strings. A string is shorthand for a list containing just that string. The code generated for the top-level expression will then be guarded by #if COND for each COND in the list. Example: a conditional struct { 'struct': 'IfStruct', 'data': { 'foo': 'int' }, 'if': ['defined(CONFIG_FOO)', 'defined(HAVE_BAR)'] } gets its generated code guarded like this: #if defined(CONFIG_FOO) #if defined(HAVE_BAR) ... generated code ... #endif /* defined(HAVE_BAR) */ #endif /* defined(CONFIG_FOO) */ Where a member can be defined with a single string value for its type, it is also possible to supply a dictionary instead with both 'type' and 'if' keys. Example: a conditional 'bar' member { 'struct': 'IfStruct', 'data': { 'foo': 'int', 'bar': { 'type': 'int', 'if': 'defined(IFCOND)'} } } An enum value can be replaced by a dictionary with a 'name' and a 'if' key. Example: a conditional 'bar' enum member. { 'enum': 'IfEnum', 'data': [ 'foo', { 'name' : 'bar', 'if': 'defined(IFCOND)' } ] } Similarly, features can be specified as a dictionary with a 'name' and an 'if' key. Example: a conditional 'allow-negative-numbers' feature { 'struct': 'TestType', 'data': { 'number': 'int' }, 'features': [ { 'name': 'allow-negative-numbers', 'if' 'defined(IFCOND)' } ] } Please note that you are responsible to ensure that the C code will compile with an arbitrary combination of conditions, since the generators are unable to check it at this point. The presence of 'if' keys in the schema is reflected through to the introspection output depending on the build configuration. === Documentation comments === A multi-line comment that starts and ends with a '##' line is a documentation comment. These are parsed by the documentation generator, which recognizes certain markup detailed below. ==== Documentation markup ==== Comment text starting with '=' is a section title: # = Section title Double the '=' for a subsection title: # == Subsection title '|' denotes examples: # | Text of the example, may span # | multiple lines '*' starts an itemized list: # * First item, may span # multiple lines # * Second item You can also use '-' instead of '*'. A decimal number followed by '.' starts a numbered list: # 1. First item, may span # multiple lines # 2. Second item The actual number doesn't matter. You could even use '*' instead of '2.' for the second item. Lists can't be nested. Blank lines are currently not supported within lists. Additional whitespace between the initial '#' and the comment text is permitted. *foo* and _foo_ are for strong and emphasis styles respectively (they do not work over multiple lines). @foo is used to reference a name in the schema. Example: ## # = Section # == Subsection # # Some text foo with *strong* and _emphasis_ # 1. with a list # 2. like that # # And some code: # | $ echo foo # | -> do this # | <- get that # ## ==== Expression documentation ==== Expressions other than include and pragma directives may be preceded by a documentation block. Such blocks are called expression documentation blocks. When documentation is required (see pragma 'doc-required'), expression documentation blocks are mandatory. The documentation block consists of a first line naming the expression, an optional overview, a description of each argument (for commands and events) or member (for structs, unions and alternates), and optional tagged sections. FIXME: the parser accepts these things in almost any order. Extensions added after the expression was first released carry a '(since x.y.z)' comment. A tagged section starts with one of the following words: "Note:"/"Notes:", "Since:", "Example"/"Examples", "Returns:", "TODO:". The section ends with the start of a new section. A 'Since: x.y.z' tagged section lists the release that introduced the expression. For example: ## # @BlockStats: # # Statistics of a virtual block device or a block backing device. # # @device: If the stats are for a virtual block device, the name # corresponding to the virtual block device. # # @node-name: The node name of the device. (since 2.3) # # ... more members ... # # Since: 0.14.0 ## { 'struct': 'BlockStats', 'data': {'*device': 'str', '*node-name': 'str', ... more members ... } } ## # @query-blockstats: # # Query the @BlockStats for all virtual block devices. # # @query-nodes: If true, the command will query all the # block nodes ... explain, explain ... (since 2.3) # # Returns: A list of @BlockStats for each virtual block devices. # # Since: 0.14.0 # # Example: # # -> { "execute": "query-blockstats" } # <- { # ... lots of output ... # } # ## { 'command': 'query-blockstats', 'data': { '*query-nodes': 'bool' }, 'returns': ['BlockStats'] } ==== Free-form documentation ==== A documentation block that isn't an expression documentation block is a free-form documentation block. These may be used to provide additional text and structuring content. == 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. While Client JSON Protocol wire compatibility should be maintained between qemu versions, we cannot make the same guarantees for introspection stability. For example, one version of qemu may provide a non-variant optional member of a struct, and a later version rework the member to instead be non-optional and associated with a variant. Likewise, one version of qemu may list a member with open-ended type 'str', and a later version could convert it to a finite set of strings via an enum type; or a member may be converted from a specific type to an alternate that represents a choice between the original type and something else. query-qmp-schema returns a JSON array of SchemaInfo objects. These objects together describe the wire ABI, as defined in the QAPI schema. There is no specified order to the SchemaInfo objects returned; a client must search for a particular name throughout the entire array to learn more about that name, but is at least guaranteed that there will be no collisions between type, command, and event names. 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 commands and events have the same name as in the QAPI schema. Command and event names are part of the wire ABI, but type names are not. Therefore, the SchemaInfo for types have auto-generated meaningless names. For readability, the examples in this section use meaningful type names instead. To examine a type, start with a command or event using it, 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", "ret-type" and "allow-oob". 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". When "allow-oob" is set, it means the command supports out-of-band execution. 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": "q_empty", "ret-type": "SchemaInfoList" } Type "q_empty" is an automatic 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. Example: the SchemaInfo for EVENT_C from section Events { "name": "EVENT_C", "meta-type": "event", "arg-type": "q_obj-EVENT_C-arg" } Type "q_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. The "members" array is in no particular order; clients must search the entire object when learning whether a particular member is supported. 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). The "variants" array is in no particular order, and is not guaranteed to list cases in the same order as the corresponding "tag" enum type. Example: the SchemaInfo for flat union BlockdevOptions from section Union types { "name": "BlockdevOptions", "meta-type": "object", "members": [ { "name": "driver", "type": "BlockdevDriver" }, { "name": "read-only", "type": "bool", "default": null } ], "tag": "driver", "variants": [ { "case": "file", "type": "BlockdevOptionsFile" }, { "case": "qcow2", "type": "BlockdevOptionsQcow2" } ] } 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). A simple union implicitly defines an object type for each of its variants. Example: the SchemaInfo for simple union BlockdevOptionsSimple from section Union types { "name": "BlockdevOptionsSimple", "meta-type": "object", "members": [ { "name": "type", "type": "BlockdevOptionsSimpleKind" } ], "tag": "type", "variants": [ { "case": "file", "type": "q_obj-BlockdevOptionsFile-wrapper" }, { "case": "qcow2", "type": "q_obj-BlockdevOptionsQcow2-wrapper" } ] } Enumeration type "BlockdevOptionsSimpleKind" and the object types "q_obj-BlockdevOptionsFile-wrapper", "q_obj-BlockdevOptionsQcow2-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. There is no guarantee on the order in which "members" will be listed. Example: the SchemaInfo for BlockdevRef from section Alternate types { "name": "BlockdevRef", "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. For convenience, the array's name may resemble the element type; however, clients should examine member "element-type" instead of making assumptions based on parsing member "name". Example: the SchemaInfo for ['str'] { "name": "[str]", "meta-type": "array", "element-type": "str" } The SchemaInfo for an enumeration type has meta-type "enum" and variant member "values". The values are listed in no particular order; clients must search the entire enum when learning whether a particular value is supported. 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 == The QAPI code generator qapi-gen.py generates code and documentation from the schema. Together with the core QAPI libraries, this code provides 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, map the response back to a Client JSON Protocol response to be returned to the user, and introspect the commands. As an example, we'll use the following schema, which describes a single complex user-defined type, along with command which takes a list of that type as a parameter, and returns a single element of that type. The user is responsible for writing the implementation of qmp_my_command(); everything else is produced by the generator. $ cat example-schema.json { 'struct': 'UserDefOne', 'data': { 'integer': 'int', '*string': 'str' } } { 'command': 'my-command', 'data': { 'arg1': ['UserDefOne'] }, 'returns': 'UserDefOne' } { 'event': 'MY_EVENT' } We run qapi-gen.py like this: $ python scripts/qapi-gen.py --output-dir="qapi-generated" \ --prefix="example-" example-schema.json For a more thorough look at generated code, the testsuite includes tests/qapi-schema/qapi-schema-tests.json that covers more examples of what the generator will accept, and compiles the resulting C code as part of 'make check-unit'. === Code generated for QAPI types === The following files are created: $(prefix)qapi-types.h - C types corresponding to types defined in the schema $(prefix)qapi-types.c - Cleanup functions for the above C types The $(prefix) is an optional parameter used as a namespace to keep the generated code from one schema/code-generation separated from others so code can be generated/used from multiple schemas without clobbering previously created code. Example: $ cat qapi-generated/example-qapi-types.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_TYPES_H #define EXAMPLE_QAPI_TYPES_H #include "qapi/qapi-builtin-types.h" typedef struct UserDefOne UserDefOne; typedef struct UserDefOneList UserDefOneList; typedef struct q_obj_my_command_arg q_obj_my_command_arg; struct UserDefOne { int64_t integer; bool has_string; char *string; }; void qapi_free_UserDefOne(UserDefOne *obj); struct UserDefOneList { UserDefOneList *next; UserDefOne *value; }; void qapi_free_UserDefOneList(UserDefOneList *obj); struct q_obj_my_command_arg { UserDefOneList *arg1; }; #endif /* EXAMPLE_QAPI_TYPES_H */ $ cat qapi-generated/example-qapi-types.c [Uninteresting stuff omitted...] void qapi_free_UserDefOne(UserDefOne *obj) { Visitor *v; if (!obj) { return; } v = qapi_dealloc_visitor_new(); visit_type_UserDefOne(v, NULL, &obj, NULL); visit_free(v); } void qapi_free_UserDefOneList(UserDefOneList *obj) { Visitor *v; if (!obj) { return; } v = qapi_dealloc_visitor_new(); visit_type_UserDefOneList(v, NULL, &obj, NULL); visit_free(v); } [Uninteresting stuff omitted...] For a modular QAPI schema (see section Include directives), code for each sub-module SUBDIR/SUBMODULE.json is actually generated into SUBDIR/$(prefix)qapi-types-SUBMODULE.h SUBDIR/$(prefix)qapi-types-SUBMODULE.c If qapi-gen.py is run with option --builtins, additional files are created: qapi-builtin-types.h - C types corresponding to built-in types qapi-builtin-types.c - Cleanup functions for the above C types === Code generated for visiting QAPI types === These are the visitor functions used to walk through and convert between a native QAPI C data structure and some other format (such as QObject); the generated functions are named visit_type_FOO() and visit_type_FOO_members(). The following files are generated: $(prefix)qapi-visit.c: Visitor function for a particular C type, used to automagically convert QObjects into the corresponding C type and vice-versa, as well as for deallocating memory for an existing C type $(prefix)qapi-visit.h: Declarations for previously mentioned visitor functions Example: $ cat qapi-generated/example-qapi-visit.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_VISIT_H #define EXAMPLE_QAPI_VISIT_H #include "qapi/qapi-builtin-visit.h" #include "example-qapi-types.h" void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp); void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp); void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp); void visit_type_q_obj_my_command_arg_members(Visitor *v, q_obj_my_command_arg *obj, Error **errp); #endif /* EXAMPLE_QAPI_VISIT_H */ $ cat qapi-generated/example-qapi-visit.c [Uninteresting stuff omitted...] void visit_type_UserDefOne_members(Visitor *v, UserDefOne *obj, Error **errp) { Error *err = NULL; visit_type_int(v, "integer", &obj->integer, &err); if (err) { goto out; } if (visit_optional(v, "string", &obj->has_string)) { visit_type_str(v, "string", &obj->string, &err); if (err) { goto out; } } out: error_propagate(errp, err); } void visit_type_UserDefOne(Visitor *v, const char *name, UserDefOne **obj, Error **errp) { Error *err = NULL; visit_start_struct(v, name, (void **)obj, sizeof(UserDefOne), &err); if (err) { goto out; } if (!*obj) { goto out_obj; } visit_type_UserDefOne_members(v, *obj, &err); if (err) { goto out_obj; } visit_check_struct(v, &err); out_obj: visit_end_struct(v, (void **)obj); if (err && visit_is_input(v)) { qapi_free_UserDefOne(*obj); *obj = NULL; } out: error_propagate(errp, err); } void visit_type_UserDefOneList(Visitor *v, const char *name, UserDefOneList **obj, Error **errp) { Error *err = NULL; UserDefOneList *tail; size_t size = sizeof(**obj); visit_start_list(v, name, (GenericList **)obj, size, &err); if (err) { goto out; } for (tail = *obj; tail; tail = (UserDefOneList *)visit_next_list(v, (GenericList *)tail, size)) { visit_type_UserDefOne(v, NULL, &tail->value, &err); if (err) { break; } } if (!err) { visit_check_list(v, &err); } visit_end_list(v, (void **)obj); if (err && visit_is_input(v)) { qapi_free_UserDefOneList(*obj); *obj = NULL; } out: error_propagate(errp, err); } void visit_type_q_obj_my_command_arg_members(Visitor *v, q_obj_my_command_arg *obj, Error **errp) { Error *err = NULL; visit_type_UserDefOneList(v, "arg1", &obj->arg1, &err); if (err) { goto out; } out: error_propagate(errp, err); } [Uninteresting stuff omitted...] For a modular QAPI schema (see section Include directives), code for each sub-module SUBDIR/SUBMODULE.json is actually generated into SUBDIR/$(prefix)qapi-visit-SUBMODULE.h SUBDIR/$(prefix)qapi-visit-SUBMODULE.c If qapi-gen.py is run with option --builtins, additional files are created: qapi-builtin-visit.h - Visitor functions for built-in types qapi-builtin-visit.c - Declarations for these visitor functions === Code generated for commands === These are the marshaling/dispatch functions for the commands defined in the schema. The generated code provides qmp_marshal_COMMAND(), and declares qmp_COMMAND() that the user must implement. The following files are generated: $(prefix)qapi-commands.c: Command marshal/dispatch functions for each QMP command defined in the schema $(prefix)qapi-commands.h: Function prototypes for the QMP commands specified in the schema Example: $ cat qapi-generated/example-qapi-commands.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_COMMANDS_H #define EXAMPLE_QAPI_COMMANDS_H #include "example-qapi-types.h" #include "qapi/qmp/dispatch.h" UserDefOne *qmp_my_command(UserDefOneList *arg1, Error **errp); void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp); void example_qmp_init_marshal(QmpCommandList *cmds); #endif /* EXAMPLE_QAPI_COMMANDS_H */ $ cat qapi-generated/example-qapi-commands.c [Uninteresting stuff omitted...] static void qmp_marshal_output_UserDefOne(UserDefOne *ret_in, QObject **ret_out, Error **errp) { Error *err = NULL; Visitor *v; v = qobject_output_visitor_new(ret_out); visit_type_UserDefOne(v, "unused", &ret_in, &err); if (!err) { visit_complete(v, ret_out); } error_propagate(errp, err); visit_free(v); v = qapi_dealloc_visitor_new(); visit_type_UserDefOne(v, "unused", &ret_in, NULL); visit_free(v); } void qmp_marshal_my_command(QDict *args, QObject **ret, Error **errp) { Error *err = NULL; UserDefOne *retval; Visitor *v; q_obj_my_command_arg arg = {0}; v = qobject_input_visitor_new(QOBJECT(args)); visit_start_struct(v, NULL, NULL, 0, &err); if (err) { goto out; } visit_type_q_obj_my_command_arg_members(v, &arg, &err); if (!err) { visit_check_struct(v, &err); } visit_end_struct(v, NULL); if (err) { goto out; } retval = qmp_my_command(arg.arg1, &err); if (err) { goto out; } qmp_marshal_output_UserDefOne(retval, ret, &err); out: error_propagate(errp, err); visit_free(v); v = qapi_dealloc_visitor_new(); visit_start_struct(v, NULL, NULL, 0, NULL); visit_type_q_obj_my_command_arg_members(v, &arg, NULL); visit_end_struct(v, NULL); visit_free(v); } void example_qmp_init_marshal(QmpCommandList *cmds) { QTAILQ_INIT(cmds); qmp_register_command(cmds, "my-command", qmp_marshal_my_command, QCO_NO_OPTIONS); } [Uninteresting stuff omitted...] For a modular QAPI schema (see section Include directives), code for each sub-module SUBDIR/SUBMODULE.json is actually generated into SUBDIR/$(prefix)qapi-commands-SUBMODULE.h SUBDIR/$(prefix)qapi-commands-SUBMODULE.c === Code generated for events === This is the code related to events defined in the schema, providing qapi_event_send_EVENT(). The following files are created: $(prefix)qapi-events.h - Function prototypes for each event type $(prefix)qapi-events.c - Implementation of functions to send an event $(prefix)qapi-emit-events.h - Enumeration of all event names, and common event code declarations $(prefix)qapi-emit-events.c - Common event code definitions Example: $ cat qapi-generated/example-qapi-events.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_EVENTS_H #define EXAMPLE_QAPI_EVENTS_H #include "qapi/util.h" #include "example-qapi-types.h" void qapi_event_send_my_event(void); #endif /* EXAMPLE_QAPI_EVENTS_H */ $ cat qapi-generated/example-qapi-events.c [Uninteresting stuff omitted...] void qapi_event_send_my_event(void) { QDict *qmp; qmp = qmp_event_build_dict("MY_EVENT"); example_qapi_event_emit(EXAMPLE_QAPI_EVENT_MY_EVENT, qmp); qobject_unref(qmp); } [Uninteresting stuff omitted...] $ cat qapi-generated/example-qapi-emit-events.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_EMIT_EVENTS_H #define EXAMPLE_QAPI_EMIT_EVENTS_H #include "qapi/util.h" typedef enum example_QAPIEvent { EXAMPLE_QAPI_EVENT_MY_EVENT, EXAMPLE_QAPI_EVENT__MAX, } example_QAPIEvent; #define example_QAPIEvent_str(val) \ qapi_enum_lookup(&example_QAPIEvent_lookup, (val)) extern const QEnumLookup example_QAPIEvent_lookup; void example_qapi_event_emit(example_QAPIEvent event, QDict *qdict); #endif /* EXAMPLE_QAPI_EMIT_EVENTS_H */ $ cat qapi-generated/example-qapi-emit-events.c [Uninteresting stuff omitted...] const QEnumLookup example_QAPIEvent_lookup = { .array = (const char *const[]) { [EXAMPLE_QAPI_EVENT_MY_EVENT] = "MY_EVENT", }, .size = EXAMPLE_QAPI_EVENT__MAX }; [Uninteresting stuff omitted...] For a modular QAPI schema (see section Include directives), code for each sub-module SUBDIR/SUBMODULE.json is actually generated into SUBDIR/$(prefix)qapi-events-SUBMODULE.h SUBDIR/$(prefix)qapi-events-SUBMODULE.c === Code generated for introspection === The following files are created: $(prefix)qapi-introspect.c - Defines a string holding a JSON description of the schema $(prefix)qapi-introspect.h - Declares the above string Example: $ cat qapi-generated/example-qapi-introspect.h [Uninteresting stuff omitted...] #ifndef EXAMPLE_QAPI_INTROSPECT_H #define EXAMPLE_QAPI_INTROSPECT_H #include "qapi/qmp/qlit.h" extern const QLitObject example_qmp_schema_qlit; #endif /* EXAMPLE_QAPI_INTROSPECT_H */ $ cat qapi-generated/example-qapi-introspect.c [Uninteresting stuff omitted...] const QLitObject example_qmp_schema_qlit = QLIT_QLIST(((QLitObject[]) { QLIT_QDICT(((QLitDictEntry[]) { { "arg-type", QLIT_QSTR("0"), }, { "meta-type", QLIT_QSTR("command"), }, { "name", QLIT_QSTR("my-command"), }, { "ret-type", QLIT_QSTR("1"), }, {} })), QLIT_QDICT(((QLitDictEntry[]) { { "arg-type", QLIT_QSTR("2"), }, { "meta-type", QLIT_QSTR("event"), }, { "name", QLIT_QSTR("MY_EVENT"), }, {} })), /* "0" = q_obj_my-command-arg */ QLIT_QDICT(((QLitDictEntry[]) { { "members", QLIT_QLIST(((QLitObject[]) { QLIT_QDICT(((QLitDictEntry[]) { { "name", QLIT_QSTR("arg1"), }, { "type", QLIT_QSTR("[1]"), }, {} })), {} })), }, { "meta-type", QLIT_QSTR("object"), }, { "name", QLIT_QSTR("0"), }, {} })), /* "1" = UserDefOne */ QLIT_QDICT(((QLitDictEntry[]) { { "members", QLIT_QLIST(((QLitObject[]) { QLIT_QDICT(((QLitDictEntry[]) { { "name", QLIT_QSTR("integer"), }, { "type", QLIT_QSTR("int"), }, {} })), QLIT_QDICT(((QLitDictEntry[]) { { "default", QLIT_QNULL, }, { "name", QLIT_QSTR("string"), }, { "type", QLIT_QSTR("str"), }, {} })), {} })), }, { "meta-type", QLIT_QSTR("object"), }, { "name", QLIT_QSTR("1"), }, {} })), /* "2" = q_empty */ QLIT_QDICT(((QLitDictEntry[]) { { "members", QLIT_QLIST(((QLitObject[]) { {} })), }, { "meta-type", QLIT_QSTR("object"), }, { "name", QLIT_QSTR("2"), }, {} })), QLIT_QDICT(((QLitDictEntry[]) { { "element-type", QLIT_QSTR("1"), }, { "meta-type", QLIT_QSTR("array"), }, { "name", QLIT_QSTR("[1]"), }, {} })), QLIT_QDICT(((QLitDictEntry[]) { { "json-type", QLIT_QSTR("int"), }, { "meta-type", QLIT_QSTR("builtin"), }, { "name", QLIT_QSTR("int"), }, {} })), QLIT_QDICT(((QLitDictEntry[]) { { "json-type", QLIT_QSTR("string"), }, { "meta-type", QLIT_QSTR("builtin"), }, { "name", QLIT_QSTR("str"), }, {} })), {} })); [Uninteresting stuff omitted...]