summaryrefslogtreecommitdiff
path: root/docs/qapi-code-gen.txt
blob: c0a9325db8224bf8f275067f6378320795689bd3 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
= 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 seperate 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
two kinds of types that are supported: complex user-defined types, and enums.

A complex type is a dictionary containing a single key who's 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.

An enumeration type is a dictionary containing a single key who's value is a
list of strings.  An example enumeration is:

 { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }

Generally speaking, complex types and enums should always use CamelCase for
the type names.

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' }

Command names should be all lower case with words separated by a hyphen.


== 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;

        visit_start_list(m, name, errp);

        for (i = visit_next_list(m, (GenericList **)obj, errp); i; i = visit_next_list(m, &i, errp)) {
            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$


=== 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$