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diff --git a/docs/devel/qom.rst b/docs/devel/qom.rst index dc5be79..0b943b2 100644 --- a/docs/devel/qom.rst +++ b/docs/devel/qom.rst @@ -2,4 +2,377 @@ The QEMU Object Model (QOM) =========================== +.. highlight:: c + +The QEMU Object Model provides a framework for registering user creatable +types and instantiating objects from those types. QOM provides the following +features: + + - System for dynamically registering types + - Support for single-inheritance of types + - Multiple inheritance of stateless interfaces + +.. code-block:: c + :caption: Creating a minimal type + + #include "qdev.h" + + #define TYPE_MY_DEVICE "my-device" + + // No new virtual functions: we can reuse the typedef for the + // superclass. + typedef DeviceClass MyDeviceClass; + typedef struct MyDevice + { + DeviceState parent; + + int reg0, reg1, reg2; + } MyDevice; + + static const TypeInfo my_device_info = { + .name = TYPE_MY_DEVICE, + .parent = TYPE_DEVICE, + .instance_size = sizeof(MyDevice), + }; + + static void my_device_register_types(void) + { + type_register_static(&my_device_info); + } + + type_init(my_device_register_types) + +In the above example, we create a simple type that is described by #TypeInfo. +#TypeInfo describes information about the type including what it inherits +from, the instance and class size, and constructor/destructor hooks. + +Alternatively several static types could be registered using helper macro +DEFINE_TYPES() + +.. code-block:: c + + static const TypeInfo device_types_info[] = { + { + .name = TYPE_MY_DEVICE_A, + .parent = TYPE_DEVICE, + .instance_size = sizeof(MyDeviceA), + }, + { + .name = TYPE_MY_DEVICE_B, + .parent = TYPE_DEVICE, + .instance_size = sizeof(MyDeviceB), + }, + }; + + DEFINE_TYPES(device_types_info) + +Every type has an #ObjectClass associated with it. #ObjectClass derivatives +are instantiated dynamically but there is only ever one instance for any +given type. The #ObjectClass typically holds a table of function pointers +for the virtual methods implemented by this type. + +Using object_new(), a new #Object derivative will be instantiated. You can +cast an #Object to a subclass (or base-class) type using +object_dynamic_cast(). You typically want to define macro wrappers around +OBJECT_CHECK() and OBJECT_CLASS_CHECK() to make it easier to convert to a +specific type: + +.. code-block:: c + :caption: Typecasting macros + + #define MY_DEVICE_GET_CLASS(obj) \ + OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE) + #define MY_DEVICE_CLASS(klass) \ + OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE) + #define MY_DEVICE(obj) \ + OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE) + +Class Initialization +==================== + +Before an object is initialized, the class for the object must be +initialized. There is only one class object for all instance objects +that is created lazily. + +Classes are initialized by first initializing any parent classes (if +necessary). After the parent class object has initialized, it will be +copied into the current class object and any additional storage in the +class object is zero filled. + +The effect of this is that classes automatically inherit any virtual +function pointers that the parent class has already initialized. All +other fields will be zero filled. + +Once all of the parent classes have been initialized, #TypeInfo::class_init +is called to let the class being instantiated provide default initialize for +its virtual functions. Here is how the above example might be modified +to introduce an overridden virtual function: + +.. code-block:: c + :caption: Overriding a virtual function + + #include "qdev.h" + + void my_device_class_init(ObjectClass *klass, void *class_data) + { + DeviceClass *dc = DEVICE_CLASS(klass); + dc->reset = my_device_reset; + } + + static const TypeInfo my_device_info = { + .name = TYPE_MY_DEVICE, + .parent = TYPE_DEVICE, + .instance_size = sizeof(MyDevice), + .class_init = my_device_class_init, + }; + +Introducing new virtual methods requires a class to define its own +struct and to add a .class_size member to the #TypeInfo. Each method +will also have a wrapper function to call it easily: + +.. code-block:: c + :caption: Defining an abstract class + + #include "qdev.h" + + typedef struct MyDeviceClass + { + DeviceClass parent; + + void (*frobnicate) (MyDevice *obj); + } MyDeviceClass; + + static const TypeInfo my_device_info = { + .name = TYPE_MY_DEVICE, + .parent = TYPE_DEVICE, + .instance_size = sizeof(MyDevice), + .abstract = true, // or set a default in my_device_class_init + .class_size = sizeof(MyDeviceClass), + }; + + void my_device_frobnicate(MyDevice *obj) + { + MyDeviceClass *klass = MY_DEVICE_GET_CLASS(obj); + + klass->frobnicate(obj); + } + +Interfaces +========== + +Interfaces allow a limited form of multiple inheritance. Instances are +similar to normal types except for the fact that are only defined by +their classes and never carry any state. As a consequence, a pointer to +an interface instance should always be of incomplete type in order to be +sure it cannot be dereferenced. That is, you should define the +'typedef struct SomethingIf SomethingIf' so that you can pass around +``SomethingIf *si`` arguments, but not define a ``struct SomethingIf { ... }``. +The only things you can validly do with a ``SomethingIf *`` are to pass it as +an argument to a method on its corresponding SomethingIfClass, or to +dynamically cast it to an object that implements the interface. + +Methods +======= + +A <emphasis>method</emphasis> is a function within the namespace scope of +a class. It usually operates on the object instance by passing it as a +strongly-typed first argument. +If it does not operate on an object instance, it is dubbed +<emphasis>class method</emphasis>. + +Methods cannot be overloaded. That is, the #ObjectClass and method name +uniquely identity the function to be called; the signature does not vary +except for trailing varargs. + +Methods are always <emphasis>virtual</emphasis>. Overriding a method in +#TypeInfo.class_init of a subclass leads to any user of the class obtained +via OBJECT_GET_CLASS() accessing the overridden function. +The original function is not automatically invoked. It is the responsibility +of the overriding class to determine whether and when to invoke the method +being overridden. + +To invoke the method being overridden, the preferred solution is to store +the original value in the overriding class before overriding the method. +This corresponds to ``{super,base}.method(...)`` in Java and C# +respectively; this frees the overriding class from hardcoding its parent +class, which someone might choose to change at some point. + +.. code-block:: c + :caption: Overriding a virtual method + + typedef struct MyState MyState; + + typedef void (*MyDoSomething)(MyState *obj); + + typedef struct MyClass { + ObjectClass parent_class; + + MyDoSomething do_something; + } MyClass; + + static void my_do_something(MyState *obj) + { + // do something + } + + static void my_class_init(ObjectClass *oc, void *data) + { + MyClass *mc = MY_CLASS(oc); + + mc->do_something = my_do_something; + } + + static const TypeInfo my_type_info = { + .name = TYPE_MY, + .parent = TYPE_OBJECT, + .instance_size = sizeof(MyState), + .class_size = sizeof(MyClass), + .class_init = my_class_init, + }; + + typedef struct DerivedClass { + MyClass parent_class; + + MyDoSomething parent_do_something; + } DerivedClass; + + static void derived_do_something(MyState *obj) + { + DerivedClass *dc = DERIVED_GET_CLASS(obj); + + // do something here + dc->parent_do_something(obj); + // do something else here + } + + static void derived_class_init(ObjectClass *oc, void *data) + { + MyClass *mc = MY_CLASS(oc); + DerivedClass *dc = DERIVED_CLASS(oc); + + dc->parent_do_something = mc->do_something; + mc->do_something = derived_do_something; + } + + static const TypeInfo derived_type_info = { + .name = TYPE_DERIVED, + .parent = TYPE_MY, + .class_size = sizeof(DerivedClass), + .class_init = derived_class_init, + }; + +Alternatively, object_class_by_name() can be used to obtain the class and +its non-overridden methods for a specific type. This would correspond to +``MyClass::method(...)`` in C++. + +The first example of such a QOM method was #CPUClass.reset, +another example is #DeviceClass.realize. + +Standard type declaration and definition macros +=============================================== + +A lot of the code outlined above follows a standard pattern and naming +convention. To reduce the amount of boilerplate code that needs to be +written for a new type there are two sets of macros to generate the +common parts in a standard format. + +A type is declared using the OBJECT_DECLARE macro family. In types +which do not require any virtual functions in the class, the +OBJECT_DECLARE_SIMPLE_TYPE macro is suitable, and is commonly placed +in the header file: + +.. code-block:: c + :caption: Declaring a simple type + + OBJECT_DECLARE_SIMPLE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE) + +This is equivalent to the following: + +.. code-block:: c + :caption: Expansion from declaring a simple type + + typedef struct MyDevice MyDevice; + typedef struct MyDeviceClass MyDeviceClass; + + G_DEFINE_AUTOPTR_CLEANUP_FUNC(MyDeviceClass, object_unref) + + #define MY_DEVICE_GET_CLASS(void *obj) \ + OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE) + #define MY_DEVICE_CLASS(void *klass) \ + OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE) + #define MY_DEVICE(void *obj) + OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE) + + struct MyDeviceClass { + DeviceClass parent_class; + }; + +The 'struct MyDevice' needs to be declared separately. +If the type requires virtual functions to be declared in the class +struct, then the alternative OBJECT_DECLARE_TYPE() macro can be +used. This does the same as OBJECT_DECLARE_SIMPLE_TYPE(), but without +the 'struct MyDeviceClass' definition. + +To implement the type, the OBJECT_DEFINE macro family is available. +In the simple case the OBJECT_DEFINE_TYPE macro is suitable: + +.. code-block:: c + :caption: Defining a simple type + + OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE) + +This is equivalent to the following: + +.. code-block:: c + :caption: Expansion from defining a simple type + + static void my_device_finalize(Object *obj); + static void my_device_class_init(ObjectClass *oc, void *data); + static void my_device_init(Object *obj); + + static const TypeInfo my_device_info = { + .parent = TYPE_DEVICE, + .name = TYPE_MY_DEVICE, + .instance_size = sizeof(MyDevice), + .instance_init = my_device_init, + .instance_finalize = my_device_finalize, + .class_size = sizeof(MyDeviceClass), + .class_init = my_device_class_init, + }; + + static void + my_device_register_types(void) + { + type_register_static(&my_device_info); + } + type_init(my_device_register_types); + +This is sufficient to get the type registered with the type +system, and the three standard methods now need to be implemented +along with any other logic required for the type. + +If the type needs to implement one or more interfaces, then the +OBJECT_DEFINE_TYPE_WITH_INTERFACES() macro can be used instead. +This accepts an array of interface type names. + +.. code-block:: c + :caption: Defining a simple type implementing interfaces + + OBJECT_DEFINE_TYPE_WITH_INTERFACES(MyDevice, my_device, + MY_DEVICE, DEVICE, + { TYPE_USER_CREATABLE }, { NULL }) + +If the type is not intended to be instantiated, then then +the OBJECT_DEFINE_ABSTRACT_TYPE() macro can be used instead: + +.. code-block:: c + :caption: Defining a simple abstract type + + OBJECT_DEFINE_ABSTRACT_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE) + + + +API Reference +------------- + .. kernel-doc:: include/qom/object.h |