# 接口 在一些设备类型定义中我们可以看到interfaces一栏。 ``` static TypeInfo pc_dimm_info = { .name = TYPE_PC_DIMM, .parent = TYPE_DEVICE, ... .interfaces = (InterfaceInfo[]) { { TYPE_MEMORY_DEVICE }, { } }, }; ``` 那这一栏是什么意思呢？让我们来进一步打开看看。 ## Interface也是一种类型 进一步看，可以发现在Qemu模型中接口也是一种类型，并且也有父子关系。 ``` static TypeInfo interface_info = { .name = TYPE_INTERFACE, .class_size = sizeof(InterfaceClass), .abstract = true, }; static const TypeInfo memory_device_info = { .name = TYPE_MEMORY_DEVICE, .parent = TYPE_INTERFACE, .class_size = sizeof(MemoryDeviceClass), }; ``` 但是和普通类型不同，他们只有对应的Class，而没有object。 我们尝试把这两个类型画成图，或许能看得更清楚一些。 ``` +---------------------------------------------------------------+ | | v | TypeInfo TypeImpl* InterfaceClass | +---------------------+ +----------------------+ +-->+------------------------+ | |name | type_new()---> |name | | |parent_class | | | TYPE_INTERFACE | | TYPE_INTERFACE | | | (ObjectClass) | | +---------------------+ |class ---|----+ | type ----|------+ | (ObjectClass*) | |concrete_class | +----------------------+ | (ObjectClass*) | |interface_type | | (TypeImpl*) | +------------------------+ | | | v +---------------------------------------------------------------+ | MemoryDeviceClass | v +--------------------------+ | TypeInfo TypeImpl* |InterfaceClass | | +---------------------+ +----------------------+ +--> | +------------------------+ | |name | type_new()---> |name | | | |parent_class | | | TYPE_MEMORY_DEVICE| | TYPE_MEMORY_DEVICE | | | | (ObjectClass) | | +---------------------+ |class ---|----+ | | type ----|---+ | (ObjectClass*) | | |concrete_class | +----------------------+ | | (ObjectClass*) | | |interface_type | | | (TypeImpl*) | | +------------------------+ |get_addr | |set_addr | |... | | | +--------------------------+ ``` ## 接口类型附属于设备类型 接口类型是一个很有意思的类型，刚才我们看到了TYPE\_MEMORY\_DEVICE会对应有一个MemoryDeviceClass。 对于普通的设备类型，那整个系统中就只会有这么一个对应的Class，但是接口类型则不是。 还是从代码上来看： ``` static void type_initialize(TypeImpl *ti) { ... for (i = 0; i < ti->num_interfaces; i++) { TypeImpl *t = type_get_by_name(ti->interfaces[i].typename); for (e = ti->class->interfaces; e; e = e->next) { TypeImpl *target_type = OBJECT_CLASS(e->data)->type; if (type_is_ancestor(target_type, t)) { break; } } if (e) { continue; } type_initialize_interface(ti, t, t); } ... } ``` 对于定义了interfaces成员的类型，在初始化中将通过type\_initialize\_interface来创建自己的接口类型。 而这个类型不仅在系统中注册，还会添加到class->interfaces链表上。 ``` +----------------------------------------------------------------+-----+ | | | | MemoryDeviceClass | | v +--------------------------+ | | TypeImpl* |InterfaceClass | | | +----------------------+ +--> | +------------------------+ | | |name | | | |parent_class | | | | TYPE_MEMORY_DEVICE | | | | (ObjectClass) | | | |class ---|----+ | | type ----|----+ | | (ObjectClass*) | | |interface_type | | +----------------------+ | | (TypeImpl*) | | ^ | |concrete_class | | | | | (ObjectClass*) | | | | +------------------------+ | | |get_addr | | | |set_addr | | | |... | | | | | | | +--------------------------+ | | | | | | | TypeImpl +----------------------------------------------------------------+ | +--------------------------+ | | | | |name | | | MemoryDeviceClass | | | TYPE_PC_DIMM | v | +--------------------------+ | | +--------------------------+ TypeImpl* | |InterfaceClass | | | ^ +----------------------+ +--> | +------------------------+ | | | |name | | | |parent_class | | | PCDIMMDeviceClass | TYPE_PC_DIMM:: | | | | (ObjectClass) | | | +--------------------------+ | TYPE_MEMORY_DEVICE | | | | type ----|----+ | |parent_class | +----->|class ---|----+ | |interface_type ----|-----------+ | (ObjectClass) | | | (ObjectClass*) | | | (TypeImpl*) | | interfaces ----|------+ +----------------------+ | |concrete_class ----|----+ | | | | (ObjectClass*) | | +--------------------------+ | +------------------------+ | ^ |get_addr | | | |set_addr | | | |... | | | | | | | +--------------------------+ | | | | | +---------------------------------------------------------------------------------------------------------+ ``` 希望这个图能够对理解接口有一点点帮助。 ## 设置接口类型的“虚函数” 接口类型重要的成员就是它的虚函数了，那谁在什么时候设置呢？ 因为接口类型是从属于设备类型的，所以虚函数的设置在设备类型初始化函数中设置。 对于pc-dimm设备，这个函数就是pc\_dimm\_class\_init中了。 ``` DeviceClass *dc = DEVICE_CLASS(oc); PCDIMMDeviceClass *ddc = PC_DIMM_CLASS(oc); MemoryDeviceClass mdc = MEMORY_DEVICE_CLASS(oc); mdc->get_addr = pc_dimm_md_get_addr; mdc->set_addr = pc_dimm_md_set_addr; ``` ## 从设备类型到接口类型 在上面这段代码中有一个有意思的地方就是怎么从设备类型PC\_DIMM\_CLASS转换到接口类型MEMORY\_DEVICE\_CLASS的 让我们来看一眼： ``` #define MEMORY_DEVICE_CLASS(klass) \ OBJECT_CLASS_CHECK(MemoryDeviceClass, (klass), TYPE_MEMORY_DEVICE) #define OBJECT_CLASS_CHECK(class_type, class, name) \ ((class_type *)object_class_dynamic_cast_assert(OBJECT_CLASS(class), (name), \ __FILE__, __LINE__, __func__)) ``` 经过这两个宏，最后会落到函数object\_class\_dynamic\_cast()。 而这个函数就是沿着klass->interfaces链表查找名字为TYPE\_MEMORY\_DEVICE的类型。如果找到了唯一的，那就范围它。 怎么样，现在是不是能看清楚这个函数的意义，以及设备类到接口类的转换了？ --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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