学步园

http://wiki.qemu.org/Features/QDevCleanup
All devices should have a DeviceState associated with them.  Otherwise, there's really no point in having qdev at all.
We have lots of devices today that don't have DeviceState's associated with them because the have a separate qdev representation with a reference to the non-DeviceState object.
We have non-DeviceState objects because otherwise we end up with an inheritance diamond.  We have this problem because we want to have relationships like: DeviceState <- SystemDeviceState <- ISADevice <- ISASerialDevice.
But ISASerialDevice is not the only type of serial device.  You can also have a SystemSerialDevice that's directly attached to the System bus.  That means you'd have to have:
SerialDevice -> ISASerialDevice -> SystemDeviceState -> DeviceState
                     -> SystemSerialDevice -> SystemDeviceState -> DeviceState
Which is a classic MI diamond.  The only way to resolve this modelling problem is to split out the common code and rely on a has-a relationship instead of an is-a.  That gives you:
ISASerialDevice->SystemDeviceState->DeviceState
SystemSerialDevice->SystemDeviceState->DeviceState
ISASerialDevice has-a SerialDevice
SystemSerialDevice has-a SerialDevice
And since we want SerialDevice inherit from a DeviceState (recall, all devices should have DeviceStates):
SerialDevice->DeviceState
No more MI diamond and all devices have DeviceStates.  Coincidentally, it matches more closely how hardware works..
Generally speaking, any time we have one device that needs to sit on multiple busses, we're going to have to model it in this fashion.