69dd62dfd6
The discriminator for anonymous unions is the data type. This allows to
have a union type that allows both of these:
{ 'file': 'my_existing_block_device_id' }
{ 'file': { 'filename': '/tmp/mydisk.qcow2', 'read-only': true } }
Unions like this are specified in the schema with an empty dict as
discriminator. For this example you could take:
{ 'union': 'BlockRef',
'discriminator': {},
'data': { 'definition': 'BlockOptions',
'reference': 'str' } }
{ 'type': 'ExampleObject',
'data: { 'file': 'BlockRef' } }
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
414 lines
14 KiB
Plaintext
414 lines
14 KiB
Plaintext
= How to use the QAPI code generator =
|
|
|
|
* Note: as of this writing, QMP does not use QAPI. Eventually QMP
|
|
commands will be converted to use QAPI internally. The following
|
|
information describes QMP/QAPI as it will exist after the
|
|
conversion.
|
|
|
|
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.
|
|
|
|
This document will describe how the schemas, scripts, and resulting
|
|
code is used.
|
|
|
|
|
|
== QMP/Guest agent schema ==
|
|
|
|
This file defines the types, commands, and events used by QMP. It should
|
|
fully describe the interface used by QMP.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
=== Complex 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:
|
|
|
|
{ 'type': 'MyType',
|
|
'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }
|
|
|
|
The use of '*' as a prefix to the name means the member is optional. Optional
|
|
members should always be added to the end of the dictionary to preserve
|
|
backwards compatibility.
|
|
|
|
=== Enumeration types ===
|
|
|
|
An enumeration type is a dictionary containing a single key whose value is a
|
|
list of strings. An example enumeration is:
|
|
|
|
{ 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
|
|
|
|
=== 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.
|
|
|
|
|
|
A simple union type defines a mapping from discriminator values to data types
|
|
like in this example:
|
|
|
|
{ 'type': 'FileOptions', 'data': { 'filename': 'str' } }
|
|
{ 'type': '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:
|
|
|
|
{ "type": "qcow2", "data" : { "backing-file": "/some/place/my-image",
|
|
"lazy-refcounts": true } }
|
|
|
|
|
|
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' } }
|
|
|
|
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 always be a string field.
|
|
The above example can then be modified as follows:
|
|
|
|
{ 'type': 'BlockdevCommonOptions',
|
|
'data': { 'driver': 'str', 'readonly': 'bool' } }
|
|
{ 'union': 'BlockdevOptions',
|
|
'base': 'BlockdevCommonOptions',
|
|
'discriminator': 'driver',
|
|
'data': { 'raw': 'RawOptions',
|
|
'qcow2': 'Qcow2Options' } }
|
|
|
|
Resulting in this JSON object:
|
|
|
|
{ "driver": "qcow2",
|
|
"readonly": false,
|
|
"backing-file": "/some/place/my-image",
|
|
"lazy-refcounts": true }
|
|
|
|
|
|
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.
|
|
|
|
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': {},
|
|
'data': { 'definition': 'BlockdevOptions',
|
|
'reference': 'str' } }
|
|
|
|
This example allows using both of the following example objects:
|
|
|
|
{ "file": "my_existing_block_device_id" }
|
|
{ "file": { "driver": "file",
|
|
"readonly": false,
|
|
'filename': "/tmp/mydisk.qcow2" } }
|
|
|
|
|
|
=== 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:
|
|
|
|
{ 'command': 'my-command',
|
|
'data': { 'arg1': 'str', '*arg2': 'str' },
|
|
'returns': 'str' }
|
|
|
|
|
|
== Code generation ==
|
|
|
|
Schemas are fed into 3 scripts to generate all the code/files that, paired
|
|
with the core QAPI libraries, comprise everything required to take JSON
|
|
commands read in by a QMP/guest agent 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.
|
|
|
|
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
|
|
node structure that can be used to chain together a list of such types in
|
|
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:
|
|
|
|
mdroth@illuin:~/w/qemu2.git$ cat example-schema.json
|
|
{ 'type': 'UserDefOne',
|
|
'data': { 'integer': 'int', 'string': 'str' } }
|
|
|
|
{ 'command': 'my-command',
|
|
'data': {'arg1': 'UserDefOne'},
|
|
'returns': 'UserDefOne' }
|
|
mdroth@illuin:~/w/qemu2.git$
|
|
|
|
=== scripts/qapi-types.py ===
|
|
|
|
Used to generate the C types defined by a schema. The following files are
|
|
created:
|
|
|
|
$(prefix)qapi-types.h - C types corresponding to types defined in
|
|
the schema you pass in
|
|
$(prefix)qapi-types.c - Cleanup functions for the above C types
|
|
|
|
The $(prefix) is an optional parameter used as a namespace to keep the
|
|
generated code from one schema/code-generation separated from others so code
|
|
can be generated/used from multiple schemas without clobbering previously
|
|
created code.
|
|
|
|
Example:
|
|
|
|
mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-types.py \
|
|
--output-dir="qapi-generated" --prefix="example-" < example-schema.json
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.c
|
|
/* AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
|
|
#include "qapi/qapi-dealloc-visitor.h"
|
|
#include "example-qapi-types.h"
|
|
#include "example-qapi-visit.h"
|
|
|
|
void qapi_free_UserDefOne(UserDefOne * obj)
|
|
{
|
|
QapiDeallocVisitor *md;
|
|
Visitor *v;
|
|
|
|
if (!obj) {
|
|
return;
|
|
}
|
|
|
|
md = qapi_dealloc_visitor_new();
|
|
v = qapi_dealloc_get_visitor(md);
|
|
visit_type_UserDefOne(v, &obj, NULL, NULL);
|
|
qapi_dealloc_visitor_cleanup(md);
|
|
}
|
|
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.h
|
|
/* AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
#ifndef QAPI_GENERATED_EXAMPLE_QAPI_TYPES
|
|
#define QAPI_GENERATED_EXAMPLE_QAPI_TYPES
|
|
|
|
#include "qapi/qapi-types-core.h"
|
|
|
|
typedef struct UserDefOne UserDefOne;
|
|
|
|
typedef struct UserDefOneList
|
|
{
|
|
UserDefOne *value;
|
|
struct UserDefOneList *next;
|
|
} UserDefOneList;
|
|
|
|
struct UserDefOne
|
|
{
|
|
int64_t integer;
|
|
char * string;
|
|
};
|
|
|
|
void qapi_free_UserDefOne(UserDefOne * obj);
|
|
|
|
#endif
|
|
|
|
|
|
=== scripts/qapi-visit.py ===
|
|
|
|
Used to generate the visitor functions used to walk through and convert
|
|
a QObject (as provided by QMP) to a native C data structure and
|
|
vice-versa, as well as the visitor function used to dealloc a complex
|
|
schema-defined C type.
|
|
|
|
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
|
|
|
|
$(prefix)qapi-visit.h: declarations for previously mentioned visitor
|
|
functions
|
|
|
|
Example:
|
|
|
|
mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-visit.py \
|
|
--output-dir="qapi-generated" --prefix="example-" < example-schema.json
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.c
|
|
/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
|
|
#include "example-qapi-visit.h"
|
|
|
|
void visit_type_UserDefOne(Visitor *m, UserDefOne ** obj, const char *name, Error **errp)
|
|
{
|
|
visit_start_struct(m, (void **)obj, "UserDefOne", name, sizeof(UserDefOne), errp);
|
|
visit_type_int(m, (obj && *obj) ? &(*obj)->integer : NULL, "integer", errp);
|
|
visit_type_str(m, (obj && *obj) ? &(*obj)->string : NULL, "string", errp);
|
|
visit_end_struct(m, errp);
|
|
}
|
|
|
|
void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp)
|
|
{
|
|
GenericList *i, **prev = (GenericList **)obj;
|
|
|
|
visit_start_list(m, name, errp);
|
|
|
|
for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
|
|
UserDefOneList *native_i = (UserDefOneList *)i;
|
|
visit_type_UserDefOne(m, &native_i->value, NULL, errp);
|
|
}
|
|
|
|
visit_end_list(m, errp);
|
|
}
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.h
|
|
/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
|
|
#ifndef QAPI_GENERATED_EXAMPLE_QAPI_VISIT
|
|
#define QAPI_GENERATED_EXAMPLE_QAPI_VISIT
|
|
|
|
#include "qapi/qapi-visit-core.h"
|
|
#include "example-qapi-types.h"
|
|
|
|
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);
|
|
|
|
#endif
|
|
mdroth@illuin:~/w/qemu2.git$
|
|
|
|
(The actual structure of the visit_type_* functions is a bit more complex
|
|
in order to propagate errors correctly and avoid leaking memory).
|
|
|
|
=== scripts/qapi-commands.py ===
|
|
|
|
Used to generate the marshaling/dispatch functions for the commands defined
|
|
in the schema. The following files are generated:
|
|
|
|
$(prefix)qmp-marshal.c: command marshal/dispatch functions for each
|
|
QMP command defined in the schema. Functions
|
|
generated by qapi-visit.py are used to
|
|
convert QObjects received from the wire into
|
|
function parameters, and uses the same
|
|
visitor functions to convert native C return
|
|
values to QObjects from transmission back
|
|
over the wire.
|
|
|
|
$(prefix)qmp-commands.h: Function prototypes for the QMP commands
|
|
specified in the schema.
|
|
|
|
Example:
|
|
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-marshal.c
|
|
/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
|
|
#include "qemu-objects.h"
|
|
#include "qapi/qmp-core.h"
|
|
#include "qapi/qapi-visit-core.h"
|
|
#include "qapi/qmp-output-visitor.h"
|
|
#include "qapi/qmp-input-visitor.h"
|
|
#include "qapi/qapi-dealloc-visitor.h"
|
|
#include "example-qapi-types.h"
|
|
#include "example-qapi-visit.h"
|
|
|
|
#include "example-qmp-commands.h"
|
|
static void qmp_marshal_output_my_command(UserDefOne * ret_in, QObject **ret_out, Error **errp)
|
|
{
|
|
QapiDeallocVisitor *md = qapi_dealloc_visitor_new();
|
|
QmpOutputVisitor *mo = qmp_output_visitor_new();
|
|
Visitor *v;
|
|
|
|
v = qmp_output_get_visitor(mo);
|
|
visit_type_UserDefOne(v, &ret_in, "unused", errp);
|
|
v = qapi_dealloc_get_visitor(md);
|
|
visit_type_UserDefOne(v, &ret_in, "unused", errp);
|
|
qapi_dealloc_visitor_cleanup(md);
|
|
|
|
|
|
*ret_out = qmp_output_get_qobject(mo);
|
|
}
|
|
|
|
static void qmp_marshal_input_my_command(QmpState *qmp__sess, QDict *args, QObject **ret, Error **errp)
|
|
{
|
|
UserDefOne * retval = NULL;
|
|
QmpInputVisitor *mi;
|
|
QapiDeallocVisitor *md;
|
|
Visitor *v;
|
|
UserDefOne * arg1 = NULL;
|
|
|
|
mi = qmp_input_visitor_new(QOBJECT(args));
|
|
v = qmp_input_get_visitor(mi);
|
|
visit_type_UserDefOne(v, &arg1, "arg1", errp);
|
|
|
|
if (error_is_set(errp)) {
|
|
goto out;
|
|
}
|
|
retval = qmp_my_command(arg1, errp);
|
|
qmp_marshal_output_my_command(retval, ret, errp);
|
|
|
|
out:
|
|
md = qapi_dealloc_visitor_new();
|
|
v = qapi_dealloc_get_visitor(md);
|
|
visit_type_UserDefOne(v, &arg1, "arg1", errp);
|
|
qapi_dealloc_visitor_cleanup(md);
|
|
return;
|
|
}
|
|
|
|
static void qmp_init_marshal(void)
|
|
{
|
|
qmp_register_command("my-command", qmp_marshal_input_my_command);
|
|
}
|
|
|
|
qapi_init(qmp_init_marshal);
|
|
mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qmp-commands.h
|
|
/* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
|
|
|
|
#ifndef QAPI_GENERATED_EXAMPLE_QMP_COMMANDS
|
|
#define QAPI_GENERATED_EXAMPLE_QMP_COMMANDS
|
|
|
|
#include "example-qapi-types.h"
|
|
#include "error.h"
|
|
|
|
UserDefOne * qmp_my_command(UserDefOne * arg1, Error **errp);
|
|
|
|
#endif
|
|
mdroth@illuin:~/w/qemu2.git$
|