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20.16.4. Controllers

Depending on the guest virtual machine architecture, it is possible to assign many virtual devices to a single bus. Under normal circumstances libvirt can automatically infer which controller to use for the bus. However, it may be necessary to provide an explicit <controller> element in the guest virtual machine XML: 

  ...
  <devices>
    <controller type='ide' index='0'/>
    <controller type='virtio-serial' index='0' ports='16' vectors='4'/>
    <controller type='virtio-serial' index='1'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
    <controller type='scsi' index='0' model='virtio-scsi' num_queues='8'/>
    </controller>
    ...
  </devices>
  ...

Figure 20.25. Controller Elements

Each controller has a mandatory attribute type, which must be one of "ide", "fdc", "scsi", "sata", "usb", "ccid", or "virtio-serial", and a mandatory attribute index which is the decimal integer describing in which order the bus controller is encountered (for use in controller attributes of address elements). The "virtio-serial" controller has two additional optional attributes, ports and vectors, which control how many devices can be connected through the controller.
A <controller type='scsi'> has an optional attribute model, which is one of "auto", "buslogic", "ibmvscsi", "lsilogic", "lsias1068", "virtio-scsi or "vmpvscsi". It should be noted that virtio-scsi controllers and drivers will work on both KVM and Windows guest virtual machines. The <controller type='scsi'> also has an attribute num_queues which enables multi-queue support for the number of queues specified.
A "usb" controller has an optional attribute model, which is one of "piix3-uhci", "piix4-uhci", "ehci", "ich9-ehci1", "ich9-uhci1", "ich9-uhci2", "ich9-uhci3", "vt82c686b-uhci", "pci-ohci" or "nec-xhci". Additionally, if the USB bus needs to be explicitly disabled for the guest virtual machine, model='none' may be used. The PowerPC64 "spapr-vio" addresses do not have an associated controller.
For controllers that are themselves devices on a PCI or USB bus, an optional sub-element address can specify the exact relationship of the controller to its master bus, with semantics given above.
USB companion controllers have an optional sub-element master to specify the exact relationship of the companion to its master controller. A companion controller is on the same bus as its master, so the companion index value should be equal. 

  ...
  <devices>
    <controller type='usb' index='0' model='ich9-ehci1'>
      <address type='pci' domain='0' bus='0' slot='4' function='7'/>
    </controller>
    <controller type='usb' index='0' model='ich9-uhci1'>
      <master startport='0'/>
      <address type='pci' domain='0' bus='0' slot='4' function='0' multifunction='on'/>
    </controller>
    ...
  </devices>
  ...

Figure 20.26. Devices - controllers - USB