qemu/docs/pci_expander_bridge.txt

1 PCI EXPANDER BRIDGE (PXB)
6 PXB is a "light-weight" host bridge in the same PCI domain
7 as the main host bridge whose purpose is to enable
8 the main host bridge to support multiple PCI root buses.
12 As opposed to PCI-2-PCI bridge's secondary bus, PXB's bus
14 (different from the main host bridge) allowing the guest OS
32  - a pxb host bridge attached to NUMA 1 with an e1000 behind it
33  - a pxb host bridge attached to NUMA 0 with an e1000 behind it
34  - a pxb host bridge not attached to any NUMA with a hard drive behind it.
46   The host bridge allows to register and query the PXB's PCI root bus in QEMU.
48 …It is a regular PCI Device that resides on the piix host-bridge bus and its bus uses the same PCI …
51   PCI-2-PCI bridge. The _PRT follows the i440fx model.
54   When adding a device to PXB it is attached to the bridge for two reasons:
55   - Using the bridge will enable hotplug support
56   - All the devices behind the bridge will use bridge's IO/MEM windows compacting