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.

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