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16.4. Configuring Device Controllers
Depending on the guest virtual machine architecture, some device buses can appear more than once, with a group of virtual devices tied to a virtual controller. Normally, libvirt can automatically infer such controllers without requiring explicit XML markup, but in some cases it is better to explicitly set a virtual controller element.
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Figure 16.15. Domain XML example for virtual controllers
Each controller has a mandatory attribute <controller type>, which must be one of:
ide
fdc
scsi
sata
usb
ccid
virtio-serial
pci
The <controller> element has a mandatory attribute <controller index> which is the decimal integer describing in which order the bus controller is encountered (for use in controller attributes of <address> elements). When <controller type ='virtio-serial'> there are two additional optional attributes (named ports and vectors), which control how many devices can be connected through the controller.
When <controller type ='scsi'>, there is an optional attribute model model, which can have the following values:
auto
buslogic
ibmvscsi
lsilogic
lsisas1068
lsisas1078
virtio-scsi
vmpvscsi
When <controller type ='usb'>, there is an optional attribute model model, which can have the following values:
piix3-uhci
piix4-uhci
ehci
ich9-ehci1
ich9-uhci1
ich9-uhci2
ich9-uhci3
vt82c686b-uhci
pci-ohci
nec-xhci
Note that if the USB bus needs to be explicitly disabled for the guest virtual machine, <model='none'> may be used. .
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 as shown in Section 16.5, “Setting Addresses for Devices”.
An optional sub-element <driver> can specify the driver-specific options. Currently, it only supports attribute queues, which specifies the number of queues for the controller. For best performance, it is recommended to specify a value matching the number of vCPUs.
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.
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Figure 16.16. Domain XML example for USB controllers
PCI controllers have an optional model attribute with the following possible values:
pci-root
pcie-root
pci-bridge
dmi-to-pci-bridge
For machine types which provide an implicit PCI bus, the pci-root controller with index='0' is auto-added and required to use PCI devices. pci-root has no address. PCI bridges are auto-added if there are too many devices to fit on the one bus provided by model='pci-root', or a PCI bus number greater than zero was specified. PCI bridges can also be specified manually, but their addresses should only see PCI buses provided by already specified PCI controllers. Leaving gaps in the PCI controller indexes might lead to an invalid configuration. The following XML example can be added to the <devices> section:
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Figure 16.17. Domain XML example for PCI bridge
For machine types which provide an implicit PCI Express (PCIe) bus (for example, the machine types based on the Q35 chipset), the pcie-root controller with index='0' is auto-added to the domain's configuration. pcie-root has also no address, but provides 31 slots (numbered 1-31) and can only be used to attach PCIe devices. In order to connect standard PCI devices on a system which has a pcie-root controller, a pci controller with model='dmi-to-pci-bridge' is automatically added. A dmi-to-pci-bridge controller plugs into a PCIe slot (as provided by pcie-root), and itself provides 31 standard PCI slots (which are not hot-pluggable). In order to have hot-pluggable PCI slots in the guest system, a pci-bridge controller will also be automatically created and connected to one of the slots of the auto-created dmi-to-pci-bridge controller; all guest devices with PCI addresses that are auto-determined by libvirt will be placed on this pci-bridge device.
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