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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

To perform these steps, you must be logged in as the root user.

  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 hardware-virtualization capable. 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 or Xen.

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:


 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. Check this file 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: cpu_mode and cpu_model.

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

 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 file 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 and compatibility if the guest is migrated to another host with slightly different host CPUs.

 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 absolutely 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: the guest can only be migrated to an exactly matching host CPU.


If your nova.conf file contains cpu_mode=custom, you can explicitly specify one of the supported named model using the cpu_model configuration option. For example, to configure the KVM guests to expose Nehalem CPUs, your nova.conf file should contain:

cpu_mode = custom
cpu_model = Nehalem
 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.

 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 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 ERRORstate, 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 correct. 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
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