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This is a standard PCI root bus (not a bridge) that can be added to a 440fx-based domain. Although it uses a PCI slot, this is *not* how it is connected into the PCI bus hierarchy, but is only used for control. Each pci-expander-bus provides 32 slots (0-31) that can accept hotplug of standard PCI devices. The usefulness of pci-expander-bus relative to a pci-bridge is that the NUMA node of the bus can be specified with the <node> subelement of <target>. This gives guest-side visibility to the NUMA node of attached devices (presuming that management apps only assign a device to a bus that has a NUMA node number matching the node number of the device on the host). Each pci-expander-bus also has a "busNr" attribute. The expander-bus itself will take the busNr specified, and all buses that are connected to this bus (including the pci-bridge that is automatically added to any expander bus of model "pxb" (see the next commit)) will use busNr+1, busNr+2, etc, and the pci-root (or the expander-bus with next lower busNr) will use bus numbers lower than busNr.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>