Re: QOM address space handling

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/12/20 23:32, Eduardo Habkost wrote:
> Who owns the FlatView reference, exactly?

The AddressSpace.  The device creates the AddressSpace, which holds a 
reference to the MemoryRegion through FlatView and AddressSpaceDispatch, 
which holds a reference to the device.

By destroying the address space that it created, the device can break 
the reference loop.

> If the FlatView reference is owned by the MemoryRegion, we have a
> reference loop: the device holds a reference to the MemoryRegion,
> which owns the FlatView, which holds a reference to the device.
> In this case, who owns the reference loop and is responsible for
> breaking it?

The reference loop is owned by the device, which breaks it through 
unrealize (called by unparent).

instance_finalize by definition cannot break reference loops, so this 
means that my suggestion of using address_space_init in instance_init 
was wrong.

Thanks,

Paolo

> If the FlatView reference is not owned by the MemoryRegion, who
> owns it?


>>>> There are 2 possible solutions here: 1) ensure QOM objects that add
>>>> address spaces during instance init have a corresponding instance
>>>> finalize function to remove them or 2) move the creation of address
>>>> spaces from instance init to realize.
>>>>
>>>> Does anyone have any arguments for which solution is preferred?
>>> 
>>> I slightly prefer (1) because there could be cases where you also create
>>> subdevices using that address space, and in order to set properties of
>>> subdevices before realize, you would have to create the subdevices in
>>> instance_init as well.