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diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index 8313ba6af8..269a1f3d27 100644 --- a/docs/qapi-code-gen.txt +++ b/docs/qapi-code-gen.txt @@ -1,61 +1,193 @@ = How to use the QAPI code generator = -QAPI is a native C API within QEMU which provides management-level -functionality to internal/external users. For external -users/processes, this interface is made available by a JSON-based -QEMU Monitor protocol that is provided by the QMP server. - -To map QMP-defined 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. The QEMU Guest Agent also uses these -scripts, paired with a separate schema, to generate -marshaling/dispatch code for the guest agent server running in the -guest. +Copyright IBM Corp. 2011 +Copyright (C) 2012-2015 Red Hat, Inc. -This document will describe how the schemas, scripts, and resulting -code are used. +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 == -== QMP/Guest agent schema == - -This file defines the types, commands, and events used by QMP. It should -fully describe the interface used by QMP. +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. -This file is designed to be loosely based on JSON although it's technically -executable Python. While dictionaries are used, they are parsed as -OrderedDicts so that ordering is preserved. +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. -There are two basic syntaxes used, type definitions and command definitions. -The first syntax defines a type and is represented by a dictionary. There are -three kinds of user-defined types that are supported: complex types, -enumeration types and union types. +== QMP/Guest agent schema == -Generally speaking, types definitions should always use CamelCase for the type -names. Command names should be all lower case with words separated by a hyphen. +A QAPI schema file is designed to be loosely based on JSON +(http://www.ietf.org/rfc/rfc7159.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 are allowed; anything between an unquoted # and the following +newline is ignored. Although there is not yet a documentation +generator, a form of stylized comments has developed for consistently +documenting details about an expression and when it was added to the +schema. The documentation is delimited between two lines of ##, then +the first line names the expression, an optional overview is provided, +then individual documentation about each member of 'data' is provided, +and finally, a 'Since: x.y.z' tag lists the release that introduced +the expression. Optional fields are tagged with the phrase +'#optional', often with their default value; and extensions added +after the expression was first released are also given a '(since +x.y.z)' comment. For example: + + ## + # @BlockStats: + # + # Statistics of a virtual block device or a block backing device. + # + # @device: #optional If the stats are for a virtual block device, the name + # corresponding to the virtual block device. + # + # @stats: A @BlockDeviceStats for the device. + # + # @parent: #optional This describes the file block device if it has one. + # + # @backing: #optional This describes the backing block device if it has one. + # (Since 2.0) + # + # Since: 0.14.0 + ## + { 'struct': 'BlockStats', + 'data': {'*device': 'str', 'stats': 'BlockDeviceStats', + '*parent': 'BlockStats', + '*backing': 'BlockStats'} } + +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 seven top-level expressions recognized by the parser: +'include', '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). + +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 +definitions should not end in 'Kind', as this namespace is used for +creating implicit C enums for visiting union types. Command names, +and field 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. The +special string '**' appears for some commands that manually perform +their own type checking rather than relying on the type-safe code +produced by the qapi code generators. + +Any name (command, event, type, field, or enum value) beginning with +"x-" is marked experimental, and may be withdrawn or changed +incompatibly in a future release. Downstream vendors may add +extensions; such extensions should begin with a prefix matching +"__RFQDN_" (for the reverse-fully-qualified-domain-name of the +vendor), even if the rest of the name uses dash (example: +__com.redhat_drive-mirror). Other than downstream extensions (with +leading underscore and the use of dots), all names should begin with a +letter, and contain only ASCII letters, digits, dash, and underscore. +It is okay to reuse names that match C keywords; the generator will +rename a field named "default" in the QAPI to "q_default" in the +generated C code. + +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 built-in to the parser: + 'str' - arbitrary UTF-8 string + 'int' - 64-bit signed integer (although the C code may place further + restrictions on acceptable range) + 'number' - floating point number + 'bool' - JSON value of true or false + 'int8', 'int16', 'int32', 'int64' - like 'int', but enforce maximum + bit size + 'uint8', 'uint16', 'uint32', 'uint64' - unsigned counterparts + 'size' - like 'uint64', but allows scaled suffix from command line + visitor === Includes === +Usage: { 'include': STRING } + The QAPI schema definitions can be modularized using the 'include' directive: - { 'include': 'path/to/file.json'} + { '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 safe. +file using the directive. Multiple includes of the same file are +safe. 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. -=== Complex types === +=== Struct types === -A complex type is a dictionary containing a single key whose value is a -dictionary. This corresponds to a struct in C or an Object in JSON. An -example of a complex type is: +Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME } - { 'type': 'MyType', +A struct is a dictionary containing a single 'data' key whose +value is a dictionary. 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. +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 @@ -84,13 +216,13 @@ 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 complex type definition can specify another complex type as its base. +A struct definition can specify another struct as its base. In this case, the fields of the base type are included as top-level fields -of the new complex type's dictionary in the QMP wire format. An example -definition is: +of the new struct's dictionary in the Client JSON Protocol wire +format. An example definition is: - { 'type': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } } - { 'type': 'BlockdevOptionsGenericCOWFormat', + { 'struct': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } } + { 'struct': 'BlockdevOptionsGenericCOWFormat', 'base': 'BlockdevOptionsGenericFormat', 'data': { '*backing': 'str' } } @@ -100,97 +232,158 @@ both fields like this: { "file": "/some/place/my-image", "backing": "/some/place/my-backing-file" } + === Enumeration types === -An enumeration type is a dictionary containing a single key whose value is a -list of strings. An example enumeration is: +Usage: { 'enum': 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 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 field that will only contain +a finite set of string values, using an enum type for that field is +better than open-coding the field to be type 'str'. + + === Union types === -Union types are used to let the user choose between several different data -types. A union type is defined using a dictionary as explained in the -following paragraphs. +Usage: { 'union': STRING, 'data': DICT } +or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME, + '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), flat (both discriminator and base). A union +type is defined using a data dictionary as explained in the following +paragraphs. -A simple union type defines a mapping from discriminator values to data types -like in this example: +A simple union type defines a mapping from automatic discriminator +values to data types like in this example: - { 'type': 'FileOptions', 'data': { 'filename': 'str' } } - { 'type': 'Qcow2Options', + { 'struct': 'FileOptions', 'data': { 'filename': 'str' } } + { 'struct': 'Qcow2Options', 'data': { 'backing-file': 'str', 'lazy-refcounts': 'bool' } } { 'union': 'BlockdevOptions', 'data': { 'file': 'FileOptions', 'qcow2': 'Qcow2Options' } } -In the QMP wire format, a simple union is represented by a dictionary that -contains the 'type' field as a discriminator, and a 'data' field that is of the -specified data type corresponding to the discriminator value: +In the Client JSON Protocol, a simple union is represented by a +dictionary that contains the 'type' field as a discriminator, and a +'data' field 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-file": "/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. No branch of +the union can be named 'max', as this would collide with the implicit +enum. The value for each branch can be of any type. -A union definition can specify a complex type as its base. In this case, the -fields of the complex type are included as top-level fields of the union -dictionary in the QMP wire format. An example definition is: - { 'type': 'BlockdevCommonOptions', 'data': { 'readonly': 'bool' } } - { 'union': 'BlockdevOptions', - 'base': 'BlockdevCommonOptions', - 'data': { 'raw': 'RawOptions', - 'qcow2': 'Qcow2Options' } } +A flat union definition specifies a struct as its base, and +avoids nesting on the wire. All branches of the union must be +complex types, and the top-level fields of the union dictionary on +the wire will be combination of fields from both the base type and the +appropriate branch type (when merging two dictionaries, there must be +no keys in common). The 'discriminator' field must be the name of an +enum-typed member of the base struct. -And it looks like this on the wire: - - { "type": "qcow2", - "readonly": false, - "data" : { "backing-file": "/some/place/my-image", - "lazy-refcounts": true } } - - -Flat union types avoid the nesting on the wire. They are used whenever a -specific field of the base type is declared as the discriminator ('type' is -then no longer generated). The discriminator must be of enumeration type. -The above example can then be modified as follows: +The following example enhances the above simple union example by +adding a common field 'readonly', renaming the discriminator to +something more applicable, and reducing the number of {} required on +the wire: { 'enum': 'BlockdevDriver', 'data': [ 'raw', 'qcow2' ] } - { 'type': 'BlockdevCommonOptions', + { 'struct': 'BlockdevCommonOptions', 'data': { 'driver': 'BlockdevDriver', 'readonly': 'bool' } } { 'union': 'BlockdevOptions', 'base': 'BlockdevCommonOptions', 'discriminator': 'driver', - 'data': { 'raw': 'RawOptions', + 'data': { 'file': 'FileOptions', 'qcow2': 'Qcow2Options' } } -Resulting in this JSON object: +Resulting in these JSON objects: + + { "driver": "file", "readonly": true, + "filename": "/some/place/my-image" } + { "driver": "qcow2", "readonly": false, + "backing-file": "/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 can ensure that branches exist for all values of the +enum (although the order of the keys need not match the declaration of +the enum). In the resulting generated C data types, a flat union is +represented as a struct with the base member fields 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, - { "driver": "qcow2", - "readonly": false, - "backing-file": "/some/place/my-image", - "lazy-refcounts": true } + { 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } } +is identical on the wire to: -A special type of unions are anonymous unions. They don't form a dictionary in -the wire format but allow the direct use of different types in their place. As -they aren't structured, they don't have any explicit discriminator but use -the (QObject) data type of their value as an implicit discriminator. This means -that they are restricted to using only one discriminator value per QObject -type. For example, you cannot have two different complex types in an anonymous -union, or two different integer types. + { 'enum': 'Enum', 'data': ['one', 'two'] } + { 'struct': 'Base', 'data': { 'type': 'Enum' } } + { 'struct': 'Branch1', 'data': { 'data': 'str' } } + { 'struct': 'Branch2', 'data': { 'data': 'int' } } + { 'union': 'Flat': 'base': 'Base', 'discriminator': 'type', + 'data': { 'one': 'Branch1', 'two': 'Branch2' } } -Anonymous unions are declared using an empty dictionary as their discriminator. -The discriminator values never appear on the wire, they are only used in the -generated C code. Anonymous unions cannot have a base type. - { 'union': 'BlockRef', - 'discriminator': {}, +=== 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': 'BlockRef', 'data': { 'definition': 'BlockdevOptions', 'reference': 'str' } } -This example allows using both of the following example objects: +Just like for a simple union, an implicit C enum 'NameKind' is created +to enumerate the branches for the alternate 'Name'. + +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; 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", @@ -200,23 +393,95 @@ This example allows using both of the following example objects: === Commands === -Commands are defined by using a list containing three members. The first -member is the command name, the second member is a dictionary containing -arguments, and the third member is the return type. - -An example command is: +Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, + '*returns': TYPE-NAME-OR-DICT, + '*gen': false, '*success-response': false } + +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, a one-element array containing the name +of a complex type, or a dictionary that declares an anonymous type +with the same semantics as a 'struct' expression, with one exception +noted below when 'gen' is used. + +The 'returns' member describes what will appear in the "return" field +of a Client JSON Protocol reply on successful completion of a command. +The member is optional from the command declaration; if absent, the +"return" field will be an empty dictionary. If 'returns' is present, +it must be the string name of a complex or built-in type, a +one-element array containing the name of a complex or built-in type, +or a dictionary that declares an anonymous type with the same +semantics as a 'struct' expression, with one exception noted below +when 'gen' is used. Although it is permitted to have the 'returns' +member name a built-in type or an array of built-in types, any command +that does this cannot be extended to return additional information in +the future; thus, new commands should strongly consider returning a +dictionary-based type or an array of dictionaries, even if the +dictionary only contains one field at the present. + +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" }, { } ] } + +In rare cases, QAPI cannot express a type-safe representation of a +corresponding Client JSON Protocol command. In these cases, if the +command expression includes the key 'gen' with boolean value false, +then the 'data' or 'returns' member that intends to bypass generated +type-safety and do its own manual validation should use an inline +dictionary definition, with a value of '**' rather than a valid type +name for the keys that the generated code will not validate. Please +try to avoid adding new commands that rely on this, and instead use +type-safe unions. For an example of bypass usage: + + { 'command': 'netdev_add', + 'data': {'type': 'str', 'id': 'str', '*props': '**'}, + 'gen': false } + +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 should include the optional key +'success-response' with boolean value false. So far, only QGA makes +use of this field. - { 'command': 'my-command', - 'data': { 'arg1': 'str', '*arg2': 'str' }, - 'returns': 'str' } === Events === -Events are defined with the keyword 'event'. When 'data' is also specified, -additional info will be included in the event. Finally there will be C API -generated in qapi-event.h; when called by QEMU code, a message with timestamp -will be emitted on the wire. If timestamp is -1, it means failure to retrieve -host time. +Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT } + +Events are defined with the keyword 'event'. It is not allowed to +name an event 'MAX', since the generator also produces a C enumeration +of all event names with a generated _MAX value at the end. 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-event.h; when called by QEMU code, a +message with timestamp will be emitted on the wire. An example event is: @@ -234,9 +499,9 @@ Resulting in this JSON object: 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 QMP/guest agent server, unmarshal the arguments into +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 QMP/guest agent response to be returned to the user. +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 @@ -245,7 +510,7 @@ case we want to accept/return a list of this type with a command), and a command which takes that type as a parameter and returns the same type: $ cat example-schema.json - { 'type': 'UserDefOne', + { 'struct': 'UserDefOne', 'data': { 'integer': 'int', 'string': 'str' } } { 'command': 'my-command', @@ -311,7 +576,7 @@ Example: #ifndef EXAMPLE_QAPI_TYPES_H #define EXAMPLE_QAPI_TYPES_H -[Builtin types omitted...] +[Built-in types omitted...] typedef struct UserDefOne UserDefOne; @@ -324,7 +589,7 @@ Example: struct UserDefOneList *next; } UserDefOneList; -[Functions on builtin types omitted...] +[Functions on built-in types omitted...] struct UserDefOne { @@ -423,7 +688,7 @@ Example: #ifndef EXAMPLE_QAPI_VISIT_H #define EXAMPLE_QAPI_VISIT_H -[Visitors for builtin types omitted...] +[Visitors for built-in types omitted...] void visit_type_UserDefOne(Visitor *m, UserDefOne **obj, const char *name, Error **errp); void visit_type_UserDefOneList(Visitor *m, UserDefOneList **obj, const char *name, Error **errp); |