Some of this is installation guide material and should probably be moved.

KVM is configured as the default hypervisor for Compute.


This document contains several sections about hypervisor selection. If you are reading this document linearly, you do not want to load the KVM module before you install nova-compute. The nova-compute service depends on qemu-kvm, which installs /lib/udev/rules.d/45-qemu-kvm.rules, which sets the correct permissions on the /dev/kvm device node.

To enable KVM explicitly, add the following configuration options to the /etc/nova/nova.conf file:

compute_driver = libvirt.LibvirtDriver

virt_type = kvm

The KVM hypervisor supports the following virtual machine image formats:

  • Raw

  • QEMU Copy-on-write (QCOW2)

  • QED Qemu Enhanced Disk

  • VMware virtual machine disk format (vmdk)

This section describes how to enable KVM on your system. For more information, see the following distribution-specific documentation:

Enable KVM

The following sections outline how to enable KVM based hardware virtualization on different architectures and platforms. To perform these steps, you must be logged in as the root user.

For x86 based systems

  1. To determine whether the svm or vmx CPU extensions are present, run this command:

    # grep -E 'svm|vmx' /proc/cpuinfo

    This command generates output if the CPU is capable of hardware-virtualization. Even if output is shown, you might still need to enable virtualization in the system BIOS for full support.

    If no output appears, consult your system documentation to ensure that your CPU and motherboard support hardware virtualization. Verify that any relevant hardware virtualization options are enabled in the system BIOS.

    The BIOS for each manufacturer is different. If you must enable virtualization in the BIOS, look for an option containing the words virtualization, VT, VMX, or SVM.

  2. To list the loaded kernel modules and verify that the kvm modules are loaded, run this command:

    # lsmod | grep kvm

    If the output includes kvm_intel or kvm_amd, the kvm hardware virtualization modules are loaded and your kernel meets the module requirements for OpenStack Compute.

    If the output does not show that the kvm module is loaded, run this command to load it:

    # modprobe -a kvm

    Run the command for your CPU. For Intel, run this command:

    # modprobe -a kvm-intel

    For AMD, run this command:

    # modprobe -a kvm-amd

    Because a KVM installation can change user group membership, you might need to log in again for changes to take effect.

    If the kernel modules do not load automatically, use the procedures listed in these subsections.

If the checks indicate that required hardware virtualization support or kernel modules are disabled or unavailable, you must either enable this support on the system or find a system with this support.


Some systems require that you enable VT support in the system BIOS. If you believe your processor supports hardware acceleration but the previous command did not produce output, reboot your machine, enter the system BIOS, and enable the VT option.

If KVM acceleration is not supported, configure Compute to use a different hypervisor, such as QEMU.

These procedures help you load the kernel modules for Intel-based and AMD-based processors if they do not load automatically during KVM installation.

Intel-based processors

If your compute host is Intel-based, run these commands as root to load the kernel modules:

# modprobe kvm
# modprobe kvm-intel

Add these lines to the /etc/modules file so that these modules load on reboot:


AMD-based processors

If your compute host is AMD-based, run these commands as root to load the kernel modules:

# modprobe kvm
# modprobe kvm-amd

Add these lines to /etc/modules file so that these modules load on reboot:


For POWER based systems

KVM as a hypervisor is supported on POWER system’s PowerNV platform.

  1. To determine if your POWER platform supports KVM based virtualization run the following command:

    # cat /proc/cpuinfo | grep PowerNV

    If the previous command generates the following output, then CPU supports KVM based virtualization.

    platform: PowerNV

    If no output is displayed, then your POWER platform does not support KVM based hardware virtualization.

  2. To list the loaded kernel modules and verify that the kvm modules are loaded, run the following command:

    # lsmod | grep kvm

    If the output includes kvm_hv, the kvm hardware virtualization modules are loaded and your kernel meets the module requirements for OpenStack Compute.

    If the output does not show that the kvm module is loaded, run the following command to load it:

    # modprobe -a kvm

    For PowerNV platform, run the following command:

    # modprobe -a kvm-hv

    Because a KVM installation can change user group membership, you might need to log in again for changes to take effect.

Configure Compute backing storage

Backing Storage is the storage used to provide the expanded operating system image, and any ephemeral storage. Inside the virtual machine, this is normally presented as two virtual hard disks (for example, /dev/vda and /dev/vdb respectively). However, inside OpenStack, this can be derived from one of these methods: lvm, qcow, rbd or flat, chosen using the libvirt.images_type option in nova.conf on the compute node.


The option raw is acceptable but deprecated in favor of flat. The Flat back end uses either raw or QCOW2 storage. It never uses a backing store, so when using QCOW2 it copies an image rather than creating an overlay. By default, it creates raw files but will use QCOW2 when creating a disk from a QCOW2 if force_raw_images is not set in configuration.

QCOW is the default backing store. It uses a copy-on-write philosophy to delay allocation of storage until it is actually needed. This means that the space required for the backing of an image can be significantly less on the real disk than what seems available in the virtual machine operating system.

Flat creates files without any sort of file formatting, effectively creating files with the plain binary one would normally see on a real disk. This can increase performance, but means that the entire size of the virtual disk is reserved on the physical disk.

Local LVM volumes can also be used. Set the libvirt.images_volume_group configuration option to the name of the LVM group you have created.

Direct download of images from Ceph

When the Glance image service is set up with the Ceph backend and Nova is using a local ephemeral store ([libvirt]/images_type!=rbd), it is possible to configure Nova to download images directly into the local compute image cache.

With the following configuration, images are downloaded using the RBD export command instead of using the Glance HTTP API. In some situations, especially for very large images, this could be substantially faster and can improve the boot times of instances.

On the Glance API node in glance-api.conf:


On the Nova compute node in nova.conf:


Specify the CPU model of KVM guests

The Compute service enables you to control the guest CPU model that is exposed to KVM virtual machines. Use cases include:

  • To maximize performance of virtual machines by exposing new host CPU features to the guest

  • To ensure a consistent default CPU across all machines, removing reliance of variable QEMU defaults

In libvirt, the CPU is specified by providing a base CPU model name (which is a shorthand for a set of feature flags), a set of additional feature flags, and the topology (sockets/cores/threads). The libvirt KVM driver provides a number of standard CPU model names. These models are defined in the /usr/share/libvirt/cpu_map.xml file for libvirt prior to version 4.7.0 or /usr/share/libvirt/cpu_map/*.xml files thereafter. Make a check to determine which models are supported by your local installation.

Two Compute configuration options in the libvirt group of nova.conf define which type of CPU model is exposed to the hypervisor when using KVM: libvirt.cpu_mode and libvirt.cpu_models.

The libvirt.cpu_mode option can take one of the following values: none, host-passthrough, host-model, and custom.

See Effective Virtual CPU configuration in Nova for a recorded presentation about this topic.

Host model (default for KVM & QEMU)

If your nova.conf file contains cpu_mode=host-model, libvirt identifies the CPU model in /usr/share/libvirt/cpu_map.xml for version prior to 4.7.0 or /usr/share/libvirt/cpu_map/*.xml for version 4.7.0 and higher that most closely matches the host, and requests additional CPU flags to complete the match. This configuration provides the maximum functionality and performance and maintains good reliability.

With regard to enabling and facilitating live migration between compute nodes, you should assess whether host-model is suitable for your compute architecture. In general, using host-model is a safe choice if your compute node CPUs are largely identical. However, if your compute nodes span multiple processor generations, you may be better advised to select a custom CPU model.

Host pass through

If your nova.conf file contains cpu_mode=host-passthrough, libvirt tells KVM to pass through the host CPU with no modifications. The difference to host-model, instead of just matching feature flags, every last detail of the host CPU is matched. This gives the best performance, and can be important to some apps which check low level CPU details, but it comes at a cost with respect to migration.

In host-passthrough mode, the guest can only be live-migrated to a target host that matches the source host extremely closely. This definitely includes the physical CPU model and running microcode, and may even include the running kernel. Use this mode only if

  • your compute nodes have a very large degree of homogeneity (i.e. substantially all of your compute nodes use the exact same CPU generation and model), and you make sure to only live-migrate between hosts with exactly matching kernel versions, or

  • you decide, for some reason and against established best practices, that your compute infrastructure should not support any live migration at all.


If nova.conf contains libvirt.cpu_mode=custom, you can explicitly specify an ordered list of supported named models using the libvirt.cpu_models configuration option. It is expected that the list is ordered so that the more common and less advanced cpu models are listed earlier.

An end user can specify required CPU features through traits. When specified, the libvirt driver will select the first cpu model in the libvirt.cpu_models list that can provide the requested feature traits. If no CPU feature traits are specified then the instance will be configured with the first cpu model in the list.

For example, if specifying CPU features avx and avx2 as follows:

$ openstack flavor set FLAVOR_ID --property trait:HW_CPU_X86_AVX=required \
                                 --property trait:HW_CPU_X86_AVX2=required

and libvirt.cpu_models is configured like this:

cpu_mode = custom
cpu_models = Penryn,IvyBridge,Haswell,Broadwell,Skylake-Client

Then Haswell, the first cpu model supporting both avx and avx2, will be chosen by libvirt.

In selecting the custom mode, along with a libvirt.cpu_models that matches the oldest of your compute node CPUs, you can ensure that live migration between compute nodes will always be possible. However, you should ensure that the libvirt.cpu_models you select passes the correct CPU feature flags to the guest.

If you need to further tweak your CPU feature flags in the custom mode, see Set CPU feature flags.


If libvirt.cpu_models is configured, the CPU models in the list needs to be compatible with the host CPU. Also, if libvirt.cpu_model_extra_flags is configured, all flags needs to be compatible with the host CPU. If incompatible CPU models or flags are specified, nova service will raise an error and fail to start.

None (default for all libvirt-driven hypervisors other than KVM & QEMU)

If your nova.conf file contains cpu_mode=none, libvirt does not specify a CPU model. Instead, the hypervisor chooses the default model.

Set CPU feature flags

Regardless of whether your selected libvirt.cpu_mode is host-passthrough, host-model, or custom, it is also possible to selectively enable additional feature flags. Suppose your selected custom CPU model is IvyBridge, which normally does not enable the pcid feature flag — but you do want to pass pcid into your guest instances. In that case, you would set:

cpu_mode = custom
cpu_models = IvyBridge
cpu_model_extra_flags = pcid

Nested guest support

You may choose to enable support for nested guests — that is, allow your Nova instances to themselves run hardware-accelerated virtual machines with KVM. Doing so requires a module parameter on your KVM kernel module, and corresponding nova.conf settings.

Nested guest support in the KVM kernel module

To enable nested KVM guests, your compute node must load the kvm_intel or kvm_amd module with nested=1. You can enable the nested parameter permanently, by creating a file named /etc/modprobe.d/kvm.conf and populating it with the following content:

options kvm_intel nested=1
options kvm_amd nested=1

A reboot may be required for the change to become effective.

Nested guest support in nova.conf

To support nested guests, you must set your libvirt.cpu_mode configuration to one of the following options:

Host pass through

In this mode, nested virtualization is automatically enabled once the KVM kernel module is loaded with nesting support.

cpu_mode = host-passthrough

However, do consider the other implications that Host pass through mode has on compute functionality.

Host model

In this mode, nested virtualization is automatically enabled once the KVM kernel module is loaded with nesting support, if the matching CPU model exposes the vmx feature flag to guests by default (you can verify this with virsh capabilities on your compute node). If your CPU model does not pass in the vmx flag, you can force it with libvirt.cpu_model_extra_flags:

cpu_mode = host-model
cpu_model_extra_flags = vmx

Again, consider the other implications that apply to the Host model (default for KVM & Qemu) mode.


In custom mode, the same considerations apply as in host-model mode, but you may additionally want to ensure that libvirt passes not only the vmx, but also the pcid flag to its guests:

cpu_mode = custom
cpu_models = IvyBridge
cpu_model_extra_flags = vmx,pcid

Nested guest support limitations

When enabling nested guests, you should be aware of (and inform your users about) certain limitations that are currently inherent to nested KVM virtualization. Most importantly, guests using nested virtualization will, while nested guests are running,

  • fail to complete live migration;

  • fail to resume from suspend.

See the KVM documentation for more information on these limitations.

AMD SEV (Secure Encrypted Virtualization)

Secure Encrypted Virtualization (SEV) is a technology from AMD which enables the memory for a VM to be encrypted with a key unique to the VM. SEV is particularly applicable to cloud computing since it can reduce the amount of trust VMs need to place in the hypervisor and administrator of their host system.

Nova supports SEV from the Train release onwards.

Requirements for SEV

First the operator will need to ensure the following prerequisites are met:

  • At least one of the Nova compute hosts must be AMD hardware capable of supporting SEV. It is entirely possible for the compute plane to be a mix of hardware which can and cannot support SEV, although as per the section on Permanent limitations below, the maximum number of simultaneously running guests with SEV will be limited by the quantity and quality of SEV-capable hardware available.

  • An appropriately configured software stack on those compute hosts, so that the various layers are all SEV ready:

    • kernel >= 4.16

    • QEMU >= 2.12

    • libvirt >= 4.5

    • ovmf >= commit 75b7aa9528bd 2018-07-06

Deploying SEV-capable infrastructure

In order for users to be able to use SEV, the operator will need to perform the following steps:

Additionally the cloud operator should consider the following optional steps:

  • Configure the libvirt.num_memory_encrypted_guests option in nova.conf to represent the number of guests an SEV compute node can host concurrently with memory encrypted at the hardware level. For example:

    num_memory_encrypted_guests = 15

    This option exists because on AMD SEV-capable hardware, the memory controller has a fixed number of slots for holding encryption keys, one per guest. For example, at the time of writing, earlier generations of hardware only have 15 slots, thereby limiting the number of SEV guests which can be run concurrently to 15. Nova needs to track how many slots are available and used in order to avoid attempting to exceed that limit in the hardware.

    At the time of writing (September 2019), work is in progress to allow QEMU and libvirt to expose the number of slots available on SEV hardware; however until this is finished and released, it will not be possible for Nova to programmatically detect the correct value.

    So this configuration option serves as a stop-gap, allowing the cloud operator the option of providing this value manually. It may later be demoted to a fallback value for cases where the limit cannot be detected programmatically, or even removed altogether when Nova’s minimum QEMU version guarantees that it can always be detected.


    When deciding whether to use the default of None or manually impose a limit, operators should carefully weigh the benefits vs. the risk. The benefits of using the default are a) immediate convenience since nothing needs to be done now, and b) convenience later when upgrading compute hosts to future versions of Nova, since again nothing will need to be done for the correct limit to be automatically imposed. However the risk is that until auto-detection is implemented, users may be able to attempt to launch guests with encrypted memory on hosts which have already reached the maximum number of guests simultaneously running with encrypted memory. This risk may be mitigated by other limitations which operators can impose, for example if the smallest RAM footprint of any flavor imposes a maximum number of simultaneously running guests which is less than or equal to the SEV limit.

  • Configure libvirt.hw_machine_type on all SEV-capable compute hosts to include x86_64=q35, so that all x86_64 images use the q35 machine type by default. (Currently Nova defaults to the pc machine type for the x86_64 architecture, although it is expected that this will change in the future.)

    Changing the default from pc to q35 makes the creation and configuration of images by users more convenient by removing the need for the hw_machine_type property to be set to q35 on every image for which SEV booting is desired.


    Consider carefully whether to set this option. It is particularly important since a limitation of the implementation prevents the user from receiving an error message with a helpful explanation if they try to boot an SEV guest when neither this configuration option nor the image property are set to select a q35 machine type.

    On the other hand, setting it to q35 may have other undesirable side-effects on other images which were expecting to be booted with pc, so it is suggested to set it on a single compute node or aggregate, and perform careful testing of typical images before rolling out the setting to all SEV-capable compute hosts.

Launching SEV instances

Once an operator has covered the above steps, users can launch SEV instances either by requesting a flavor for which the operator set the hw:mem_encryption extra spec to True, or by using an image with the hw_mem_encryption property set to True.

These do not inherently cause a preference for SEV-capable hardware, but for now SEV is the only way of fulfilling the requirement for memory encryption. However in the future, support for other hardware-level guest memory encryption technology such as Intel MKTME may be added. If a guest specifically needs to be booted using SEV rather than any other memory encryption technology, it is possible to ensure this by adding trait:HW_CPU_X86_AMD_SEV=required to the flavor extra specs or image properties.

In all cases, SEV instances can only be booted from images which have the hw_firmware_type property set to uefi, and only when the machine type is set to q35. This can be set per image by setting the image property hw_machine_type=q35, or per compute node by the operator via libvirt.hw_machine_type as explained above.

Impermanent limitations

The following limitations may be removed in the future as the hardware, firmware, and various layers of software receive new features:

  • SEV-encrypted VMs cannot yet be live-migrated or suspended, therefore they will need to be fully shut down before migrating off an SEV host, e.g. if maintenance is required on the host.

  • SEV-encrypted VMs cannot contain directly accessible host devices (PCI passthrough). So for example mdev vGPU support will not currently work. However technologies based on vhost-user should work fine.

  • The boot disk of SEV-encrypted VMs can only be virtio. (virtio-blk is typically the default for libvirt disks on x86, but can also be explicitly set e.g. via the image property hw_disk_bus=virtio). Valid alternatives for the disk include using hw_disk_bus=scsi with hw_scsi_model=virtio-scsi , or hw_disk_bus=sata.

  • QEMU and libvirt cannot yet expose the number of slots available for encrypted guests in the memory controller on SEV hardware. Until this is implemented, it is not possible for Nova to programmatically detect the correct value. As a short-term workaround, operators can optionally manually specify the upper limit of SEV guests for each compute host, via the new libvirt.num_memory_encrypted_guests configuration option described above.

Permanent limitations

The following limitations are expected long-term:


For the sake of eliminating any doubt, the following actions are not expected to be limited when SEV encryption is used:

  • Cold migration or shelve, since they power off the VM before the operation at which point there is no encrypted memory (although this could change since there is work underway to add support for PMEM)

  • Snapshot, since it only snapshots the disk

  • nova evacuate (despite the name, more akin to resurrection than evacuation), since this is only initiated when the VM is no longer running

  • Attaching any volumes, as long as they do not require attaching via an IDE bus

  • Use of spice / VNC / serial / RDP consoles

  • VM guest virtual NUMA (a.k.a. vNUMA)

For further technical details, see the nova spec for SEV support.

Guest agent support

Use guest agents to 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.

KVM performance tweaks

The VHostNet kernel module improves network performance. To load the kernel module, run the following command as root:

# modprobe vhost_net

Troubleshoot KVM

Trying to launch a new virtual machine instance fails with the ERROR state, and the following error appears in the /var/log/nova/nova-compute.log file:

libvirtError: internal error no supported architecture for os type 'hvm'

This message indicates that the KVM kernel modules were not loaded.

If you cannot start VMs after installation without rebooting, the permissions might not be set correctly. This can happen if you load the KVM module before you install nova-compute. To check whether the group is set to kvm, run:

# ls -l /dev/kvm

If it is not set to kvm, run:

# udevadm trigger