1PCI EXPANDER BRIDGE (PXB) 2========================= 3 4Description 5=========== 6PXB is a "light-weight" host bridge in the same PCI domain 7as the main host bridge whose purpose is to enable 8the main host bridge to support multiple PCI root buses. 9It is implemented only for i440fx and can be placed only 10on bus 0 (pci.0). 11 12As opposed to PCI-2-PCI bridge's secondary bus, PXB's bus 13is a primary bus and can be associated with a NUMA node 14(different from the main host bridge) allowing the guest OS 15to recognize the proximity of a pass-through device to 16other resources as RAM and CPUs. 17 18Usage 19===== 20A detailed command line would be: 21 22[qemu-bin + storage options] 23-m 2G 24-object memory-backend-ram,size=1024M,policy=bind,host-nodes=0,id=ram-node0 -numa node,nodeid=0,cpus=0,memdev=ram-node0 25-object memory-backend-ram,size=1024M,policy=bind,host-nodes=1,id=ram-node1 -numa node,nodeid=1,cpus=1,memdev=ram-node1 26-device pxb,id=bridge1,bus=pci.0,numa_node=1,bus_nr=4 -netdev user,id=nd -device e1000,bus=bridge1,addr=0x4,netdev=nd 27-device pxb,id=bridge2,bus=pci.0,numa_node=0,bus_nr=8 -device e1000,bus=bridge2,addr=0x3 28-device pxb,id=bridge3,bus=pci.0,bus_nr=40 -drive if=none,id=drive0,file=[img] -device virtio-blk-pci,drive=drive0,scsi=off,bus=bridge3,addr=1 29 30Here you have: 31 - 2 NUMA nodes for the guest, 0 and 1. (both mapped to the same NUMA node in host, but you can and should put it in different host NUMA nodes) 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. 35 36Limitations 37=========== 38Please observe that we specified the bus "pci.0" for the second and third pxb. 39This is because when no bus is given, another pxb can be selected by QEMU as default bus, 40however, PXBs can be placed only under the root bus. 41 42Implementation 43============== 44The PXB is composed by: 45- HostBridge (TYPE_PXB_HOST) 46 The host bridge allows to register and query the PXB's PCI root bus in QEMU. 47- PXBDev(TYPE_PXB_DEVICE) 48 It is a regular PCI Device that resides on the piix host-bridge bus and its bus uses the same PCI domain. 49 However, the bus behind is exposed through ACPI as a primary PCI bus and starts a new PCI hierarchy. 50 The interrupts from devices behind the PXB are routed through this device the same as if it were a 51 PCI-2-PCI bridge. The _PRT follows the i440fx model. 52- PCIBridgeDev(TYPE_PCI_BRIDGE_DEV) 53 Created automatically as part of init sequence. 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 57 the PCI address space. 58 59