Re: [libvirt] new libvirt "pci" controller type and pcie/q35 (was Re: [PATCH 4/7] add pci-bridge controller type)

On 04/05/2013 03:26 PM, Alex Williamson wrote: I think we're starting to get closer to the concrete problem that's bothering me. As I understand it (and again - "what I understand" has repeatedly been shown to be incorrect in this thread :-): * Ihere are multiple different types of devices that provide a bus with 1 or more "slots" that PCI devices (e.g., the virtio-net-pci device, the e1000 network device, etc) can be plugged into. * In the config for those devices, there is a required (auto-generated if not explicitly provided) <address> element that indicates what controller that device is plugged into e.g.: <interface type='direct'> ... <address type='pci' domain='0' bus='0' slot='3' function='0'/> ... </interface> * domain is always hardcoded to 0, and in the past bus was also always hardcoded to 0 because until now there has only been a single place where PCI devices could be connected - the builtin pci.0 bus, which is a part of the basic "pc" (and some others) virtual machine and provides 32 slots. * Now we are adding the ability to define new PCI buses, for now just a single kind - a pci-bridge controller, which itself must connect to an existing PCI slot, and provides 32 new PCI slots. But in the future there will be more different types of controllers that provide one or more PCI slots where PCI devices/controllers can be plugged in. * In these patches adding support for pci-bridge, we are making the assumption that there is a 1:1 correspondence between the "index='n'" attribute of the pci-bridge controller and the "bus='n'" attribute of the <address> element in devices that will be plugged into that controller. So for example if we have: <controller type='pci-bridge' index='1'> <address type='pci' domain='0' bus='0' slot='10' function='0'/> </controller> and then change the <interface> definition above to say "bus='1'", that interface device will plug into this new bus at slot 3. * So let's assume that we add a new controller called "dmi-to-pci-bridge": <controller type='dmi-to-pci-bridge' index='0'/> Ignoring for now the question of what address we give in the definition of *this* device (which is itself problematic - do we need a new "pcie" address type?), if some device is then defined with <address type='pci bus='0' .../> How do we differentiate between that meaning "the pci-ptp controller that is index='0'" and "the pci-bridge controller that is index='0'"? Do we need to expand our <address> element further? If, as I think you suggest, we have multiple different kinds of controllers that provide PCI slots, each with its own namespace, the current pci address element is inadequate to unambiguously describe where a pci device should be plugged in.