On Mon, Mar 22, 2010 at 03:51:43PM +0000, Paul Brook wrote: > > > It's a classic OOP problem. > > > > > > VirtIOBlock is-a VirtIODevice, VirtIODevice is-a DeviceState > > > > > > VirtIOPCI is-a PCIDevice, PCIDevice is-a Device State. > > > > > > But VirtIODevice is-a VirtIOPCI device isn't always true so it can't be > > > an is-a relationship. Initially, this was true and that's why the code > > > was structured that way. Now that we have two type so of virtio > > > transports, we need to change the modelling. It needs to get inverted > > > into a has-a relationship. IOW, VirtIOPCI has-a VirtIODevice. > > > > > > When one device has-a one or more other devices, we model that as a Bus. > > > > Hmm. Is anything wrong with VirtIOPCIBlock which would be both a > > VirtIOBlock and VirtIOPCI device? > > You need to solve multiple inheritance with common (but divergent) ancestors. > > A single device may not have more than one DeviceState. i.e. the DeviceState > that is part of the VirtIOBlock must be the same as the DeviceState that is > part of the PCIDevice. However VirtIOBlock and PCIDevice can not know about > each other or about VirtIOPCIBlock. > > In the current code VirtIOPCIBlock already exists, though only for the > purposes of device creation/configuration. The remainder of the code and data > structures have clean separation between PCI and Virtio code.
We can have VirtIOPCIBlock have a single DeviceState. > Anthony's suggestion is that we turn VirtIODevice into a user visible entity > (rather than an implementation detail). This makes the hoist bindings (virtio- > pci.c) completely independent of the virtio backends (e.g. virtio-block.c). > VirtIOPCIBlock ceases to exist, except maybe as a helpful alias to create a > VirtioPCI[Proxy]/VirtioBlock pair. > > > > It's just like SCSI. SCSIDisk is-a SCSIDevice, SCSIDevice is-a > > > DeviceState. > > > > > > LSIState is-a PCIDevice, PCIDevice is-a DeviceState. > > > > > > LSIState has-a SCSIDevice because LSIState implements the SCSIBus > > > interface. > > > > Yes but LSIState emulates a real HBI ... > > Not relevant. There should be nothing magic about virtio. If there is then > we're doing it wrong. > > Paul
