The current PCI subsystem has kind of half-hearted support for multiple independent root buses - aka PCI domains - in the form of the PCIHostBus structure and its domain field. However, it doesn't quite work because pci_host_bus_register() is always called with a domain of 0.
Worse, though, the whole concept of numbered domains isn't general enough. Many platforms can have independent root buses (usually on wholly independent host bridges), but only x86 gives them a hardware-significant domain number, essentially as a hack to allow all the separate config spaces to be accessed via the same IO ports. Linux guests on other platforms will show domain numbers in lspci, but these are purely guest assigned, so qemu won't know about them. This patch series, therefore, removes the broken-as-is domain concept from qemu, and replaces it with a different way of handling multiple root buses, based on a host bridge class method to provide a identifier for the root bus. This hook is designed in such a way as to allow a single bridge object to support mutiple root buses with future changes, which will allow future implementations of x86 north bridges with multiple domains to be supported correctly, and in way that matches the existing practice for all external interfaces.
