Host device management

Libvirt provides management of both physical and virtual host devices (historically also referred to as node devices) like USB, PCI, SCSI, and network devices. This also includes various virtualization capabilities which the aforementioned devices provide for utilization, for example SR-IOV, NPIV, DRM, etc.

The node device driver provides means to list and show details about host devices (virsh nodedev-list, virsh nodedev-dumpxml), which are generic and can be used with all devices. It also provides means to create and destroy devices (virsh nodedev-create, virsh nodedev-destroy) which are meant to be used to create virtual devices, currently only supported by NPIV (more info about NPIV)). Devices on the host system are arranged in a tree-like hierarchy, with the root node being called computer. The node device driver supports two backends to manage the devices, HAL and udev, with the former being deprecated in favour of the latter.

The generic format of a host device XML can be seen below. To identify a device both within the host and the device tree hierarchy, the following elements are used:

name: The device's name will be generated by libvirt using the subsystem, like pci and the device's sysfs basename.
path: Fully qualified sysfs path to the device.
parent: This element identifies the parent node in the device hierarchy. The value of the element will correspond with the device parent's name element or computer if the device does not have any parent.
driver: This elements reports the driver in use for this device. The presence of this element in the output XML depends on whether the underlying device manager (most likely udev) exposes information about the driver.
capability: Describes the device in terms of feature support. The element has one mandatory attribute type the value of which determines the type of the device. Currently recognized values for the attribute are: system, pci, usb, usb_device, net, scsi, scsi_host (Since 0.4.7), fc_host, vports, scsi_target (Since 0.7.3), storage (Since 1.0.4), scsi_generic (Since 1.0.7), drm (Since 3.1.0), and This element can be nested in which case it further specifies a device's capability. Refer to specific device types to see more values for the type attribute which are exclusive.

Basic structure of a node device

<device>
  <name>pci_0000_00_17_0</name>
  <path>/sys/devices/pci0000:00/0000:00:17.0</path>
  <parent>computer</parent>
  <driver>
    <name>ahci</name>
  </driver>
  <capability type='pci'>
...
  </capability>
</device>

PCI host devices

capability: When used as top level element, the supported values for the type attribute are pci and phys_function (see SR-IOV below).

<device>
  <name>pci_0000_04_00_1</name>
  <path>/sys/devices/pci0000:00/0000:00:06.0/0000:04:00.1</path>
  <parent>pci_0000_00_06_0</parent>
  <driver>
    <name>igb</name>
  </driver>
  <capability type='pci'>
    <domain>0</domain>
    <bus>4</bus>
    <slot>0</slot>
    <function>1</function>
    <product id='0x10c9'>82576 Gigabit Network Connection</product>
    <vendor id='0x8086'>Intel Corporation</vendor>
    <iommuGroup number='15'>
      <address domain='0x0000' bus='0x04' slot='0x00' function='0x1'/>
    </iommuGroup>
    <numa node='0'/>
    <pci-express>
      <link validity='cap' port='1' speed='2.5' width='2'/>
      <link validity='sta' speed='2.5' width='2'/>
    </pci-express>
  </capability>
</device>

The XML format for a PCI device stays the same for any further capabilities it supports, a single nested <capability> element will be included for each capability the device supports.

SR-IOV capability

Single root input/output virtualization (SR-IOV) allows sharing of the PCIe resources by multiple virtual environments. That is achieved by slicing up a single full-featured physical resource called physical function (PF) into multiple devices called virtual functions (VFs) sharing their configuration with the underlying PF. Despite the SR-IOV specification, the amount of VFs that can be created on a PF varies among manufacturers.

Suppose the NIC above was also SR-IOV capable, it would also include a nested <capability> element enumerating all virtual functions available on the physical device (physical port) like in the example below.

<capability type='pci'>
...
  <capability type='virt_functions' maxCount='7'>
    <address domain='0x0000' bus='0x04' slot='0x10' function='0x1'/>
    <address domain='0x0000' bus='0x04' slot='0x10' function='0x3'/>
    <address domain='0x0000' bus='0x04' slot='0x10' function='0x5'/>
    <address domain='0x0000' bus='0x04' slot='0x10' function='0x7'/>
    <address domain='0x0000' bus='0x04' slot='0x11' function='0x1'/>
    <address domain='0x0000' bus='0x04' slot='0x11' function='0x3'/>
    <address domain='0x0000' bus='0x04' slot='0x11' function='0x5'/>
  </capability>
...
</capability>

A SR-IOV child device on the other hand, would then report its top level capability type as a phys_function instead:

<device>
...
  <capability type='phys_function'>
    <address domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
  </capability>
...
<device>