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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/qapi-code-gen.txt | 237 |
1 files changed, 232 insertions, 5 deletions
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index b917962..f321d4b 100644 --- a/docs/qapi-code-gen.txt +++ b/docs/qapi-code-gen.txt @@ -502,13 +502,206 @@ Resulting in this JSON object: "timestamp": { "seconds": 1267020223, "microseconds": 435656 } } +== 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. + +query-qmp-schema returns a JSON array of SchemaInfo objects. These +objects together describe the wire ABI, as defined in the QAPI schema. + +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 entities defined in the QAPI schema have the same name +as in the schema. This is the case for all commands and events, and +most types. + +Command and event names are part of the wire ABI, but type names are +not. Therefore, looking up a type by its name in the QAPI schema is +wrong. Look up the command or event, 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" and "ret-type". 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". + +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": ":empty", "ret-type": "SchemaInfoList" } + + Type ":empty" is an 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 called ":obj-NAME-arg", +where NAME is the command or event's name. + +Example: the SchemaInfo for EVENT_C from section Events + + { "name": "EVENT_C", "meta-type": "event", + "arg-type": ":obj-EVENT_C-arg" } + + Type ":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. + +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). + +Example: the SchemaInfo for flat union BlockdevOptions from section +Union types + + { "name": "BlockdevOptions", "meta-type": "object", + "members": [ + { "name": "driver", "type": "BlockdevDriver" }, + { "name": "readonly", "type": "bool"} ], + "tag": "driver", + "variants": [ + { "case": "file", "type": "FileOptions" }, + { "case": "qcow2", "type": "Qcow2Options" } ] } + +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). +Such a type's name is made by appending "Kind" to the simple union's +name. + +A simple union implicitly defines an object type for each of its +variants. The type's name is ":obj-NAME-wrapper", where NAME is the +name of the name of the variant's type. + +Example: the SchemaInfo for simple union BlockdevOptions from section +Union types + + { "name": "BlockdevOptions", "meta-type": "object", + "members": [ + { "name": "kind", "type": "BlockdevOptionsKind" } ], + "tag": "type", + "variants": [ + { "case": "file", "type": ":obj-FileOptions-wrapper" }, + { "case": "qcow2", "type": ":obj-Qcow2Options-wrapper" } ] } + + Enumeration type "BlockdevOptionsKind" and the object types + ":obj-FileOptions-wrapper", ":obj-Qcow2Options-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. + +Example: the SchemaInfo for BlockRef from section Alternate types + + { "name": "BlockRef", "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. An array type's name is made by +appending "List" to its element type's name. + +Example: the SchemaInfo for ['str'] + + { "name": "strList", "meta-type": "array", + "element-type": "str" } + +The SchemaInfo for an enumeration type has meta-type "enum" and +variant member "values". + +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 == -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 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 Client JSON Protocol response to be returned to the user. +Schemas are fed into four 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 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 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 @@ -856,3 +1049,37 @@ Example: extern const char *const example_QAPIEvent_lookup[]; #endif + +=== scripts/qapi-introspect.py === + +Used to generate the introspection C code for a schema. The following +files are created: + +$(prefix)qmp-introspect.c - Defines a string holding a JSON + description of the schema. +$(prefix)qmp-introspect.h - Declares the above string. + +Example: + + $ python scripts/qapi-introspect.py --output-dir="qapi-generated" + --prefix="example-" example-schema.json + $ cat qapi-generated/example-qmp-introspect.c +[Uninteresting stuff omitted...] + + const char example_qmp_schema_json[] = "[" + "{\"arg-type\": \":empty\", \"meta-type\": \"event\", \"name\": \"MY_EVENT\"}, " + "{\"json-type\": \"int\", \"meta-type\": \"builtin\", \"name\": \"int\"}, " + "{\"json-type\": \"string\", \"meta-type\": \"builtin\", \"name\": \"str\"}, " + "{\"members\": [], \"meta-type\": \"object\", \"name\": \":empty\"}, " + "{\"members\": [{\"name\": \"arg1\", \"type\": \"UserDefOne\"}], \"meta-type\": \"object\", \"name\": \":obj-my-command-arg\"}, " + "{\"members\": [{\"name\": \"integer\", \"type\": \"int\"}, {\"name\": \"string\", \"type\": \"str\"}], \"meta-type\": \"object\", \"name\": \"UserDefOne\"}, " + "{\"arg-type\": \":obj-my-command-arg\", \"meta-type\": \"command\", \"name\": \"my-command\", \"ret-type\": \"UserDefOne\"}]"; + $ cat qapi-generated/example-qmp-introspect.h +[Uninteresting stuff omitted...] + + #ifndef EXAMPLE_QMP_INTROSPECT_H + #define EXAMPLE_QMP_INTROSPECT_H + + extern const char example_qmp_schema_json[]; + + #endif |
