622f557f5a
This introduces a new 'base' key for struct definitions that refers to another struct type. On the JSON level, the fields of the base type are included directly into the same namespace as the fields of the defined type, like with unions. On the C level, a pointer to a struct of the base type is included. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
431 lines
15 KiB
Plaintext
431 lines
15 KiB
Plaintext
= How to use the QAPI code generator =
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* Note: as of this writing, QMP does not use QAPI. Eventually QMP
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commands will be converted to use QAPI internally. The following
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information describes QMP/QAPI as it will exist after the
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conversion.
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QAPI is a native C API within QEMU which provides management-level
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functionality to internal/external users. For external
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users/processes, this interface is made available by a JSON-based
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QEMU Monitor protocol that is provided by the QMP server.
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To map QMP-defined interfaces to the native C QAPI implementations,
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a JSON-based schema is used to define types and function
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signatures, and a set of scripts is used to generate types/signatures,
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and marshaling/dispatch code. The QEMU Guest Agent also uses these
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scripts, paired with a separate schema, to generate
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marshaling/dispatch code for the guest agent server running in the
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guest.
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This document will describe how the schemas, scripts, and resulting
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code is used.
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== QMP/Guest agent schema ==
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This file defines the types, commands, and events used by QMP. It should
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fully describe the interface used by QMP.
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This file is designed to be loosely based on JSON although it's technically
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executable Python. While dictionaries are used, they are parsed as
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OrderedDicts so that ordering is preserved.
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There are two basic syntaxes used, type definitions and command definitions.
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The first syntax defines a type and is represented by a dictionary. There are
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three kinds of user-defined types that are supported: complex types,
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enumeration types and union types.
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Generally speaking, types definitions should always use CamelCase for the type
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names. Command names should be all lower case with words separated by a hyphen.
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=== Complex types ===
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A complex type is a dictionary containing a single key whose value is a
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dictionary. This corresponds to a struct in C or an Object in JSON. An
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example of a complex type is:
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{ 'type': 'MyType',
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'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }
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The use of '*' as a prefix to the name means the member is optional. Optional
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members should always be added to the end of the dictionary to preserve
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backwards compatibility.
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A complex type definition can specify another complex type as its base.
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In this case, the fields of the base type are included as top-level fields
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of the new complex type's dictionary in the QMP wire format. An example
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definition is:
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{ 'type': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } }
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{ 'type': 'BlockdevOptionsGenericCOWFormat',
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'base': 'BlockdevOptionsGenericFormat',
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'data': { '*backing': 'str' } }
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An example BlockdevOptionsGenericCOWFormat object on the wire could use
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both fields like this:
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{ "file": "/some/place/my-image",
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"backing": "/some/place/my-backing-file" }
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=== Enumeration types ===
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An enumeration type is a dictionary containing a single key whose value is a
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list of strings. An example enumeration is:
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{ 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
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=== Union types ===
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Union types are used to let the user choose between several different data
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types. A union type is defined using a dictionary as explained in the
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following paragraphs.
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A simple union type defines a mapping from discriminator values to data types
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like in this example:
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{ 'type': 'FileOptions', 'data': { 'filename': 'str' } }
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{ 'type': 'Qcow2Options',
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'data': { 'backing-file': 'str', 'lazy-refcounts': 'bool' } }
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{ 'union': 'BlockdevOptions',
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'data': { 'file': 'FileOptions',
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'qcow2': 'Qcow2Options' } }
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In the QMP wire format, a simple union is represented by a dictionary that
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contains the 'type' field as a discriminator, and a 'data' field that is of the
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specified data type corresponding to the discriminator value:
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{ "type": "qcow2", "data" : { "backing-file": "/some/place/my-image",
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"lazy-refcounts": true } }
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A union definition can specify a complex type as its base. In this case, the
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fields of the complex type are included as top-level fields of the union
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dictionary in the QMP wire format. An example definition is:
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{ 'type': 'BlockdevCommonOptions', 'data': { 'readonly': 'bool' } }
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{ 'union': 'BlockdevOptions',
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'base': 'BlockdevCommonOptions',
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'data': { 'raw': 'RawOptions',
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'qcow2': 'Qcow2Options' } }
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And it looks like this on the wire:
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{ "type": "qcow2",
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"readonly": false,
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"data" : { "backing-file": "/some/place/my-image",
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"lazy-refcounts": true } }
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Flat union types avoid the nesting on the wire. They are used whenever a
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specific field of the base type is declared as the discriminator ('type' is
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then no longer generated). The discriminator must always be a string field.
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The above example can then be modified as follows:
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{ 'type': 'BlockdevCommonOptions',
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'data': { 'driver': 'str', 'readonly': 'bool' } }
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{ 'union': 'BlockdevOptions',
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'base': 'BlockdevCommonOptions',
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'discriminator': 'driver',
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'data': { 'raw': 'RawOptions',
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'qcow2': 'Qcow2Options' } }
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Resulting in this JSON object:
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{ "driver": "qcow2",
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"readonly": false,
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"backing-file": "/some/place/my-image",
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"lazy-refcounts": true }
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A special type of unions are anonymous unions. They don't form a dictionary in
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the wire format but allow the direct use of different types in their place. As
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they aren't structured, they don't have any explicit discriminator but use
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the (QObject) data type of their value as an implicit discriminator. This means
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that they are restricted to using only one discriminator value per QObject
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type. For example, you cannot have two different complex types in an anonymous
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union, or two different integer types.
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Anonymous unions are declared using an empty dictionary as their discriminator.
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The discriminator values never appear on the wire, they are only used in the
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generated C code. Anonymous unions cannot have a base type.
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{ 'union': 'BlockRef',
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'discriminator': {},
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'data': { 'definition': 'BlockdevOptions',
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'reference': 'str' } }
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This example allows using both of the following example objects:
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{ "file": "my_existing_block_device_id" }
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{ "file": { "driver": "file",
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"readonly": false,
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'filename': "/tmp/mydisk.qcow2" } }
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=== Commands ===
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Commands are defined by using a list containing three members. The first
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member is the command name, the second member is a dictionary containing
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arguments, and the third member is the return type.
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An example command is:
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{ 'command': 'my-command',
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'data': { 'arg1': 'str', '*arg2': 'str' },
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'returns': 'str' }
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== Code generation ==
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Schemas are fed into 3 scripts to generate all the code/files that, paired
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with the core QAPI libraries, comprise everything required to take JSON
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commands read in by a QMP/guest agent server, unmarshal the arguments into
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the underlying C types, call into the corresponding C function, and map the
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response back to a QMP/guest agent response to be returned to the user.
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As an example, we'll use the following schema, which describes a single
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complex user-defined type (which will produce a C struct, along with a list
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node structure that can be used to chain together a list of such types in
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case we want to accept/return a list of this type with a command), and a
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command which takes that type as a parameter and returns the same type:
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mdroth@illuin:~/w/qemu2.git$ cat example-schema.json
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{ 'type': 'UserDefOne',
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'data': { 'integer': 'int', 'string': 'str' } }
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{ 'command': 'my-command',
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'data': {'arg1': 'UserDefOne'},
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'returns': 'UserDefOne' }
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mdroth@illuin:~/w/qemu2.git$
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=== scripts/qapi-types.py ===
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Used to generate the C types defined by a schema. The following files are
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created:
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$(prefix)qapi-types.h - C types corresponding to types defined in
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the schema you pass in
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$(prefix)qapi-types.c - Cleanup functions for the above C types
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The $(prefix) is an optional parameter used as a namespace to keep the
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generated code from one schema/code-generation separated from others so code
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can be generated/used from multiple schemas without clobbering previously
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created code.
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Example:
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mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-types.py \
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--output-dir="qapi-generated" --prefix="example-" < example-schema.json
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.c
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/* AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#include "qapi/qapi-dealloc-visitor.h"
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#include "example-qapi-types.h"
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#include "example-qapi-visit.h"
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void qapi_free_UserDefOne(UserDefOne * obj)
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{
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QapiDeallocVisitor *md;
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Visitor *v;
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if (!obj) {
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return;
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}
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md = qapi_dealloc_visitor_new();
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v = qapi_dealloc_get_visitor(md);
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visit_type_UserDefOne(v, &obj, NULL, NULL);
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qapi_dealloc_visitor_cleanup(md);
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}
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.h
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/* AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#ifndef QAPI_GENERATED_EXAMPLE_QAPI_TYPES
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#define QAPI_GENERATED_EXAMPLE_QAPI_TYPES
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#include "qapi/qapi-types-core.h"
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typedef struct UserDefOne UserDefOne;
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typedef struct UserDefOneList
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{
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UserDefOne *value;
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struct UserDefOneList *next;
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} UserDefOneList;
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struct UserDefOne
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{
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int64_t integer;
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char * string;
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};
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void qapi_free_UserDefOne(UserDefOne * obj);
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#endif
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=== scripts/qapi-visit.py ===
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Used to generate the visitor functions used to walk through and convert
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a QObject (as provided by QMP) to a native C data structure and
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vice-versa, as well as the visitor function used to dealloc a complex
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schema-defined C type.
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The following files are generated:
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$(prefix)qapi-visit.c: visitor function for a particular C type, used
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to automagically convert QObjects into the
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corresponding C type and vice-versa, as well
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as for deallocating memory for an existing C
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type
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$(prefix)qapi-visit.h: declarations for previously mentioned visitor
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functions
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Example:
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mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-visit.py \
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--output-dir="qapi-generated" --prefix="example-" < example-schema.json
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.c
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/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#include "example-qapi-visit.h"
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void visit_type_UserDefOne(Visitor *m, UserDefOne ** obj, const char *name, Error **errp)
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{
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visit_start_struct(m, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), errp);
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visit_type_int(m, (obj && *obj) ? &(*obj)->integer : NULL, "integer", errp);
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visit_type_str(m, (obj && *obj) ? &(*obj)->string : NULL, "string", errp);
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visit_end_struct(m, errp);
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}
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void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp)
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{
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GenericList *i, **prev = (GenericList **)obj;
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visit_start_list(m, name, errp);
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for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
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UserDefOneList *native_i = (UserDefOneList *)i;
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visit_type_UserDefOne(m, &native_i->value, NULL, errp);
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}
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visit_end_list(m, errp);
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}
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.h
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/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#ifndef QAPI_GENERATED_EXAMPLE_QAPI_VISIT
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#define QAPI_GENERATED_EXAMPLE_QAPI_VISIT
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#include "qapi/qapi-visit-core.h"
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#include "example-qapi-types.h"
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void visit_type_UserDefOne(Visitor *m, UserDefOne ** obj, const char *name, Error **errp);
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void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp);
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#endif
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mdroth@illuin:~/w/qemu2.git$
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(The actual structure of the visit_type_* functions is a bit more complex
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in order to propagate errors correctly and avoid leaking memory).
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=== scripts/qapi-commands.py ===
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Used to generate the marshaling/dispatch functions for the commands defined
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in the schema. The following files are generated:
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$(prefix)qmp-marshal.c: command marshal/dispatch functions for each
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QMP command defined in the schema. Functions
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generated by qapi-visit.py are used to
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convert QObjects received from the wire into
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function parameters, and uses the same
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visitor functions to convert native C return
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values to QObjects from transmission back
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over the wire.
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$(prefix)qmp-commands.h: Function prototypes for the QMP commands
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specified in the schema.
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Example:
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-marshal.c
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/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#include "qemu-objects.h"
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#include "qapi/qmp-core.h"
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#include "qapi/qapi-visit-core.h"
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#include "qapi/qmp-output-visitor.h"
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#include "qapi/qmp-input-visitor.h"
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#include "qapi/qapi-dealloc-visitor.h"
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#include "example-qapi-types.h"
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#include "example-qapi-visit.h"
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#include "example-qmp-commands.h"
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static void qmp_marshal_output_my_command(UserDefOne * ret_in, QObject **ret_out, Error **errp)
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{
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QapiDeallocVisitor *md = qapi_dealloc_visitor_new();
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QmpOutputVisitor *mo = qmp_output_visitor_new();
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Visitor *v;
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v = qmp_output_get_visitor(mo);
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visit_type_UserDefOne(v, &ret_in, "unused", errp);
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v = qapi_dealloc_get_visitor(md);
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visit_type_UserDefOne(v, &ret_in, "unused", errp);
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qapi_dealloc_visitor_cleanup(md);
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*ret_out = qmp_output_get_qobject(mo);
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}
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static void qmp_marshal_input_my_command(QmpState *qmp__sess, QDict *args, QObject **ret, Error **errp)
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{
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UserDefOne * retval = NULL;
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QmpInputVisitor *mi;
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QapiDeallocVisitor *md;
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Visitor *v;
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UserDefOne * arg1 = NULL;
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mi = qmp_input_visitor_new(QOBJECT(args));
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v = qmp_input_get_visitor(mi);
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visit_type_UserDefOne(v, &arg1, "arg1", errp);
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if (error_is_set(errp)) {
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goto out;
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}
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retval = qmp_my_command(arg1, errp);
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qmp_marshal_output_my_command(retval, ret, errp);
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out:
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md = qapi_dealloc_visitor_new();
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v = qapi_dealloc_get_visitor(md);
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visit_type_UserDefOne(v, &arg1, "arg1", errp);
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qapi_dealloc_visitor_cleanup(md);
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return;
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}
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static void qmp_init_marshal(void)
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{
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qmp_register_command("my-command", qmp_marshal_input_my_command);
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}
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qapi_init(qmp_init_marshal);
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mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-commands.h
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/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
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#ifndef QAPI_GENERATED_EXAMPLE_QMP_COMMANDS
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#define QAPI_GENERATED_EXAMPLE_QMP_COMMANDS
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#include "example-qapi-types.h"
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#include "error.h"
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UserDefOne * qmp_my_command(UserDefOne * arg1, Error **errp);
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#endif
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mdroth@illuin:~/w/qemu2.git$
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