Re: [Qemu-devel] [PATCH v2 00/18] qom: dynamic properties and composition tree (v2)

[Anthony Liguori]

On 12/05/2011 08:50 AM, Peter Maydell wrote:
> On 5 December 2011 14:36, Anthony Liguori<address@hidden>  wrote:
> > struct LSIDevice {
> >     PCIDevice parent;
> > };
> >
> > static void lsi_command_complete(SCSIBus *bus, SCSIRequest *req)
> > {
> >    LSIDevice *dev = LSI_DEVICE(bus);
> >    ...
> > }
>
> What is the LSI_DEVICE macro actually doing here? I assume
> it's not just a cast...

https://github.com/aliguori/qemu/blob/qom-next/hw/object.c#L376

It's quite literally dynamic_cast<LSIDevice>(bus) in C++.

> > static void lsi_scsi_bus_initfn(Interface *iface)
> > {
> >    SCSIBus *bus = SCSI_BUS(iface);
> >
> >    bus->command_complete = lsi_command_complete;
> > }
> >
> > TypeInfo lsi_device_info = {
> >   .name = TYPE_LSI,
> >   .parent = TYPE_PCI_DEVICE,
> >   .interfaces = (Interface[]){
> >      {
> >         .name = TYPE_SCSI_BUS,
> >         .interface_initfn = lsi_scsi_bus_initfn,
> >      }, {
> >      }
> >   },
> > };
> >
> > type_register_static(&lsi_device_info);
> >
> >
> > > Perhaps hidden with some macro that lets me just write
> > > SCSI_BUS_INTERFACE(dev), but that's the idea; such a lookup function is
> > > pretty much what all object models do. GObject has
> > > G_TYPE_INSTANCE_GET_INTERFACE, COM/XPCOM has QueryInterface, etc.
> > >
> > > If I understood everything so far, then here is my question. Are
> > > interfaces properties?
> >
> >
> > No.  A device is-a interface.  Hopefully the above example will make it more
> > clear.
>
> Saying a device is-a interface doesn't match reality. Devices
> have multiple interfaces with the rest of the world.

There are two ways a device can interact with the rest of the world.  It can 
expose a portion of it's functionality (such as an IRQ) via a child object. 
This is how it would expose MemoryRegions too.

You can take a subset of the exposed children (and perhaps some mapping logic), 
and for an ad-hoc interface.

But sometimes, you want the entire device to act like a specific thing.  In this 
case, you want the LSIDevice to act like SCSIBus.  Interfaces are just a more 
formal form of what would otherwise be an ad-hoc interface.

> (This is
> one of the major reasons why SysBus exists: it provides a suboptimal
> but usuable model of this for the two most common kinds of interface,
> MMIO regions and random gpio.)

My expectation is that most things that use SysBus today would not implement any 
interfaces.  They would just expose child properties.

Regards,

Anthony Liguori


> -- PMM