Other libvirt features

The libvirt driver supports a large number of additional features that don’t warrant their own section. These are gathered here.

Guest agent support

Guest agents enable optional access between compute nodes and guests through a socket, using the QMP protocol.

To enable this feature, you must set hw_qemu_guest_agent=yes as a metadata parameter on the image you wish to use to create the guest-agent-capable instances from. You can explicitly disable the feature by setting hw_qemu_guest_agent=no in the image metadata.

Watchdog behavior

Changed in version 15.0.0: (Ocata)

Add support for the disabled option.

A virtual watchdog device can be used to keep an eye on the guest server and carry out a configured action if the server hangs. The watchdog uses the i6300esb device (emulating a PCI Intel 6300ESB). Watchdog behavior can be configured using the hw:watchdog_action flavor extra spec or equivalent image metadata property. If neither the extra spec not the image metadata property are specified, the watchdog is disabled.

For example, to enable the watchdog and configure it to forcefully reset the guest in the event of a hang, run:

$ openstack flavor set $FLAVOR --property hw:watchdog_action=reset


Watchdog behavior set using the image metadata property will override behavior set using the flavor extra spec.

Random number generator

Changed in version 21.0.0: (Ussuri)

Random number generators are now enabled by default for instances.

Operating systems require good sources of entropy for things like cryptographic software. If a random-number generator device has been added to the instance through its image properties, the device can be enabled and configured using the hw_rng:allowed, hw_rng:rate_bytes and hw_rng:rate_period flavor extra specs.

To configure for example a byte rate of 5 bytes per period and a period of 1000 mSec (1 second), run:

$ openstack flavor set $FLAVOR \
    --property hw_rng:rate_bytes=5 \
    --property hw_rng:rate_period=1000

Alternatively, to disable the random number generator, run:

$ openstack flavor set $FLAVOR --property hw_rng:allowed=false

The presence of separate byte rate and rate period configurables is intentional. As noted in the QEMU docs, a smaller rate and larger period minimizes the opportunity for malicious guests to starve other guests of entropy but at the cost of responsiveness. Conversely, larger rates and smaller periods will increase the burst rate but at the potential cost of warping resource consumption in favour of a greedy guest.

Performance Monitoring Unit (vPMU)

Added in version 20.0.0: (Train)

If nova is deployed with the libvirt virt driver and libvirt.virt_type is set to qemu or kvm, a virtual performance monitoring unit (vPMU) can be enabled or disabled for an instance using the hw:pmu flavor extra spec or hw_pmu image metadata property. If the vPMU is not explicitly enabled or disabled via the flavor or image, its presence is left to QEMU to decide.

For example, to explicitly disable the vPMU, run:

$ openstack flavor set FLAVOR-NAME --property hw:pmu=false

The vPMU is used by tools like perf in the guest to provide more accurate information for profiling application and monitoring guest performance. For real time workloads, the emulation of a vPMU can introduce additional latency which would be undesirable. If the telemetry it provides is not required, the vPMU can be disabled. For most workloads the default of unset (enabled) will be correct.

Hiding hypervisor signature

Added in version 18.0.0: (Rocky)

Changed in version 21.0.0: (Ussuri)

Prior to the Ussuri release, this was called hide_hypervisor_id. An alias is provided to provide backwards compatibility.

Some hypervisors add a signature to their guests. While the presence of the signature can enable some paravirtualization features on the guest, it can also have the effect of preventing some drivers from loading. You can hide this signature by setting the hw:hide_hypervisor_id to true.

For example, to hide your signature from the guest OS, run:

$ openstack flavor set $FLAVOR --property hw:hide_hypervisor_id=true

Locked memory allocation

Added in version 26.0.0: (Zed)

Locking memory marks the guest memory allocations as unmovable and unswappable. It is implicitly enabled in a number of cases such as SEV or realtime guests but can also be enabled explicitly using the hw:locked_memory extra spec (or use hw_locked_memory image property). hw:locked_memory (also hw_locked_memory image property) accept boolean values in string format like ‘true’ or ‘false’ value. It will raise FlavorImageLockedMemoryConflict exception if both flavor and image property are specified but with different boolean values. This will only be allowed if you have also set hw:mem_page_size, so we can ensure that the scheduler can actually account for this correctly and prevent out of memory events. Otherwise, will raise LockMemoryForbidden exception.

$ openstack flavor set FLAVOR-NAME \
    --property hw:locked_memory=BOOLEAN_VALUE


This is currently only supported by the libvirt driver.