docs: Add PXB documentation

PCI EXPANDER BRIDGE (PXB)

=========================

Description

===========

PXB is a "light-weight" host bridge in the same PCI domain

as the main host bridge whose purpose is to enable

the main host bridge to support multiple PCI root buses.

It is implemented only for i440fx and can be placed only

on bus 0 (pci.0).

As opposed to PCI-2-PCI bridge's secondary bus, PXB's bus

is a primary bus and can be associated with a NUMA node

(different from the main host bridge) allowing the guest OS

to recognize the proximity of a pass-through device to

other resources as RAM and CPUs.

Usage

=====

A detailed command line would be:

[qemu-bin + storage options]

-m 2G

-object memory-backend-ram,size=1024M,policy=bind,host-nodes=0,id=ram-node0 -numa node,nodeid=0,cpus=0,memdev=ram-node0

-object memory-backend-ram,size=1024M,policy=bind,host-nodes=1,id=ram-node1 -numa node,nodeid=1,cpus=1,memdev=ram-node1

-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

-device pxb,id=bridge2,bus=pci.0,numa_node=0,bus_nr=8,bus=pci.0 -device e1000,bus=bridge2,addr=0x3

-device pxb,id=bridge3,bus=pci.0,bus_nr=40,bus=pci.0 -drive if=none,id=drive0,file=[img] -device virtio-blk-pci,drive=drive0,scsi=off,bus=bridge3,addr=1

Here you have:

 - 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)

 - a pxb host bridge attached to NUMA 1 with an e1000 behind it

 - a pxb host bridge attached to NUMA 0 with an e1000 behind it

 - a pxb host bridge not attached to any NUMA with a hard drive behind it.

Limitations

===========

Please observe that we specified the bus "pci.0" for the second and third pxb.

This is because when no bus is given, another pxb can be selected by QEMU as default bus,

however, PXBs can be placed only under the root bus.

Implementation

==============

The PXB is composed by:

- HostBridge (TYPE_PXB_HOST)

  The host bridge allows to register and query the PXB's rPCI root bus in QEMU.

- PXBDev(TYPE_PXB_DEVICE)

  It is a regular PCI Device that resides on the piix host-bridge bus and its bus uses the same PCI domain.

  However, the bus behind is exposed through ACPI as a primary PCI bus and starts a new PCI hierarchy.

  The interrupts from devices behind the PXB are routed through this device the same as if it were a

  PCI-2-PCI bridge. The _PRT follows the i440fx model.

- PCIBridgeDev(TYPE_PCI_BRIDGE_DEV)

  Created automatically as part of init sequence.

  When adding a device to PXB it is attached to the bridge for two reasons:

  - Using the bridge will enable hotplug support

  - All the devices behind the bridge will use bridge's IO/MEM windows compacting

    the PCI address space.