Re: [Qemu-devel] [RFC][PATCH 0/21] QEMU Object Model

[Paolo Bonzini <pbonzini@redhat.com>]

On 07/26/2011 04:02 PM, Anthony Liguori wrote:
>> Also because there is no hierarchy, composition in host devices can be
>> done very easily. A decorator for char/block devices, such as a "tee"
>> device, can treat the wrapped object(s) the same independent of the
>> actual class. A simple vtable works very well. GObject would also do
>> well, unifying the introspection at the cost of significantly more
>> boilerplate.
>
> The polymorphism model of QOM is identical to GObject so I'm not sure
> what you mean here.

GObject instead of QOM (just so that we have something that is already 
written).

> In the case of tee, it's just an object with two sockets.

Yes, understood.

> I have PCI patches, but didn't post them in the series. Here's how it
> works:
>
> The PCI host controller, the i440fx, has 32 sockets of PCIDevice.
> PCIDevice is a base class.

And as such it can add data members.  But when a device is on two buses, 
you cannot have both of them adding data members.  I know MI is hard to 
get right, and in fact I'm not proposing to do MI---not even interface 
inheritance.  I don't want to have any base class but DeviceState.

> The PCI host controller implements a PCIBus interface. The PCIDevices
> have a socket of a PCIBus
>
> Connecting a PCIDevice to the host bus involves setting the socket on
> the PCI host controller with the PCIDevice and then setting the
> PCIDevice's bus socket with the host controller.
>
> A PCIDevice can also be a PCIBus by implementing the PCIBus interface.
> This is what enables a PCI bridge to make sense in this model.
>
> If you're interested, the tree that has this is
> http://repo.or.cz/w/qemu/aliguori.git/tree/qdev2:/devices

Yes, this is pretty much what I had imagined.  But it does not scale to 
a topology where you have two parents, both of which want to add data 
members.

>> 1) in a flexible manner, so that it can express complex topologies (as
>> long as "plugs" and "sockets" have the same shape of course);
>
> Right, this is what we do today in QOM. Plugs and Sockets are typed.
> Those types can be interfaces or base classes so there's a lot of
> flexibility.

Interfaces (is-a) are less flexible than embedded objects (has-a).

> There are no properties of the socket.
>
> If you look at something like adding a PCI device in qdev, you add a
> child and set properties of the child to identify how the device sits on
> the PCI bus.
>
> I'd characterize this as awkward, at best. The slot index is not a
> property of the device, it's a property of how the device is connected
> to the PCI bus.

Yes, for a PCI address I agree.  But in a (parallel) SCSI bus, the LUN 
is logically a property of the device.  Same as IDE when you used to set 
jumpers to choose master/slave.  Or ISA interrupt lines.

Once you have something like this for a device that bridges two buses, 
interfaces require a lot of boilerplate for useless getters/setters.

> i440fx->slots[3] = mydevice
>
> Likewise, if slot 4 contains a PCI-to-PCI bridge that ends up being bus
> 1, and you want to assign to bus 1, slot 2, fn 0:
>
> i440fx->slots[4]->slots[2] = myotherdevice;
>
> Now you may observe that this is awkward compared to saying "bus 1".

No, I have no problem with that. :)

> The same applies equally to IDE.
>
> ide->primary.master = disk1;
> ide->secondary.master = cdrom;

For IDE, an equally good model would be:

     ide->primary.add(disk1);
     disk1.masterSlave = MASTER;
     ide->secondary.add(cdrom);
     cdrom.masterSlave = MASTER;

>> 5) convert buses to compound properties. Rather than inheriting from
>> PCIDevice, a PCI device would inherit straight from DeviceState and
>> include a PCIDevice struct that defines the backlink from a device to
>> its parent. Note that since we're using C, this is not a big change from
>> what we're doing now! (Inheritance by containment is a special case of
>> containment.) And it allows to define very flexibly a device that would
>> have to sit on two or more buses in the current qdev model. More
>> importantly, it keeps the effectiveness of the qbus ops model, while
>> removing the constraint of a tree topology.
>
> Interfaces are the right way to do this. Getting MI right is fairly hard

But we don't need is-a, we need has-a.  Multiple is-a is harder than 
single is-a.  Multiple has-a does not add any complication.

> I think all of the requirements you've outlined are currently handled in
> QOM.

They more than likely are.  The question is whether they're handled in 
the most programmer-efficient manner, and whether the advantages of a 
single grand unified object model for host and guest devices is worth 
the effort.

Paolo