Features/QOM

Naming in QDev

QDev has three namespaces: the device namespace, the bus namespace, and property namespaces.

The device namespace contains the names of qdev devices. qdev supports the ability to have anonymous devices. Anonymous devices are usually created through composition and are anonymous because the user controls the device namespace and the user has no way of allocating names for devices created through composition.

The bus namespace is parallel to the device namespace. Unlike the device namespace, busses cannot be anonymous. For busses that are created as a result of composition, a name is derived either from the device name or via the type name.

In qdev, implicit bus names are not considered stable and may change across invocations and/or versions of QEMU.

Property namespaces are local to devices. The 'id' property is reserved to refer to the name of the device. Property names do not reference child devices.

Paths cannot be meaningfully constructed in QDev. Devices can only be addressed directly by their name as there is no stable way to refer to busses under a device, or children under a bus.

Naming in QOM

In QOM, there are only two namespaces, the device namespace and the property namespace.

All devices have unique names. There are no exceptions. Devices created through composition are given unique names by deriving the name based on the parent device name and a special separator, "::", that cannot be used in user supplied names.

Since a bus is-a device in QOM, there is no notion of having multiple busses under the same device. A device can implement multiple bus interfaces, but can only be a single bus of any given bus interface.

Device names are completely independent of pathnames. For devices that are no user created, device names should be treated as opaque blobs with absolutely no semantic meaning.

All device relationships are identified as named properties. A QOM path name consists of a named device, followed by a series of properties which may or may not refer to other devices. For instance, all of the following are valid paths:

/i440fx/piix3/i8042/aux
/i440fx/slot[1.0]/i8042/aux
/i440fx/slot[1.0]/bus/piix3/i8042/aux

All of these path names are interpreted as follows:

def resolve_pathname(pathname):
    root, props = pathname[1:].split('/')
    dev = find_device(root)
    for prop in props:
        device_name = get_property(dev, prop)
        dev = find_device(device_name)
    return dev

In this specific example, the i440fx object has two properties that both refer to the PIIX3 device. The 'piix3' device is a property that reflects a device composition relationship. The 'slot[1.0]' property represents a device backlink relationship.

The PIIX3 device has a 'i8042' property based on device composition of the PC Keyboard Controller device. It also has a device backlink property, 'bus', that points to the bus that it sits on (which is the 'i440fx' object).

Finally, the PC Keyboard Controller device has an 'aux' property which is a device backlink property that can point to a PS/2 Mouse device.

The full set of devices names and properties used in this example are below:

Type: I440FX
Is-a: Device
Implements: PciBus
Name: i440fx
Properties:
 piix3: Composition<PIIX3>
 slot[1.0]: Backlink<PciDevice>

Type: PIIX3
Isa-a: PciDevice
Implements: IsaBus
Name: i440fx::piix3
Properties:
 i8042: Composition<I8042>
 bus: Backlink<PciDevice> (inherited from PciDevice)

Type: I8042
Isa-a: Device
Implements: Ps2Controller
Name: i440fx::piix3::i8042
Properties:
 aux: Backlink<Ps2Mouse>

TODO

1. Eliminate anonymous devices.
 a. Will require touching any place in the tree that creates a qdev object and
    giving a meaningful unique name.

2. Refactor any device that creates 2 or more busses to only create a single
   bus. This will mean using composition.
 a. An example: IDE controller creates two busses, one for the primary and one
    for the secondary.  Instead, IDE device should have two IDE controller sub
    devices, each device having a single bus.

3. Unify the bus/device namespaces 
 a. This is a compatibility breaker
 b. Depends on (1) and (2)

4. Modify qdev properties to allow devices to register setters/getters that use
   visitors to access properties.
 a. Implement existing properties in terms of visitors

5. Modify qdev properties to be stored in the object, not in the class

6. Expose children as named properties
 a. Read only to start with

7. Change qdev to use QOM typing
 a. Depends on (3)
 b. Must change all init functions to use QOM init functions
 c. Change all DO_UPCASTS to QOM macro casts
 d. Can be largely done via sed

8. Change children to be based on plug and socket properties
 a. Eliminate children list in each device
 b. Compatibility breaker

9. Improve object model
 a. Compatibility breaker