MemoryBackend类层次结构
作为一个设备类型，MemoryBackend自然也有自己的类层次结构。
经过这么多代码的洗礼，我就不多说什么了，直接上类继承关系图。
+------------------+ +----------------------+
| | | |
| ObjectClass | ------------------------------ | Object |
| class_init | | |
| | | |
+------------------+ +----------------------+
| |
| |
| |
v v
+--------------------------+ +----------------------+
| | | |
|HostMemoryBackendClass | ------------------------------------ | HostMemoryBackend |
| class_init | host_memory_backend_class_init | instance_init | host_memory_backend_init
| | | |
| +-----------------------+ | |
| |UserCreatableClass | | |
| | complete | host_memory_backend_memory_complete | |
+--+-----------------------+ +---+------------------+
| |
| |
| |
| |
| TYPE_MEMORY_BACKEND_RAM |
| +-----------------------+ | +----------------------+
| | | | | |
+--- |HostMemoryBackendClass | +--- |HostMemoryBackend |
| | class_init | ram_backend_class_init | | instance_init |
| | bc->alloc | ram_backend_memory_alloc | | |
| +-----------------------+ | +----------------------+
| |
| TYPE_MEMORY_BACKEND_FILE |
| +-----------------------+ | +----------------------+
| | | | | |
+--- |HostMemoryBackendClass | +--- |HostMemoryBackendFile |
| | class_init | file_backend_class_init | | instance_init |
| | bc->alloc | file_backend_memory_alloc | | |
| +-----------------------+ | +----------------------+
| |
| TYPE_MEMORY_BACKEND_RAM |
| +-----------------------+ | +----------------------+
| | | | | |
+--- |HostMemoryBackendClass | +--- |HostMemoryBackendMemfd|
| class_init | memfd_backend_class_init | instance_init | memfd_backend_instance_init
| bc->alloc | memfd_backend_memory_alloc | |
+-----------------------+ +----------------------+可以看到，现在的MemoryBackend一共有三种具体的实现 ram, file, memfd。他们大同小异，主要的区别就在于bc->alloc函数的实现不同。
那这个bc->alloc究竟是如何起作用的，请看下节[初始化流程][/memory_backend/02-init_flow.md]
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