# KVM

KVM is configured as the default hypervisor for Compute.

Note

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.

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:

- Fedora: Virtualization Getting Started Guide

- Ubuntu: KVM/Installation

- Debian: KVM Guide

- Red Hat Enterprise Linux (RHEL): Getting started with virtualization

- openSUSE: Setting Up a KVM VM Host Server

- SLES: Virtualization with KVM.

## Configuration

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

    [DEFAULT]
    compute_driver = libvirt.LibvirtDriver

    [libvirt]
    virt_type = kvm

## 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.

Note

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:

    kvm
    kvm-intel

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:

    kvm
    kvm-amd

### 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.

### For AArch64-based systems

Todo

Populate this section.

## 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.

Note

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`:

    [DEFAULT]
    show_image_direct_url=true

On the Nova compute node in nova.conf:

    [glance]
    enable_rbd_download=true
    rbd_user=glance
    rbd_pool=images
    rbd_ceph_conf=/etc/ceph/ceph.conf
    rbd_connect_timeout=5

## 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.

### Host configuration

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.

### Nova configuration

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

Host passthrough (`host-passthrough`)

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

      [libvirt]
      cpu_mode = host-passthrough

  However, do consider the other implications that host passthrough mode has on compute functionality.

Host model (`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 `svm` or `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 `svm` or `vmx` flag, you can force it with `libvirt.cpu_model_extra_flags`:

      [libvirt]
      cpu_mode = host-model

      # For AMD CPUs
      cpu_model_extra_flags = svm

      # For Intel CPUs
      cpu_model_extra_flags = vmx

  Again, consider the other implications that apply to the host model mode.

Custom (`custom`)

  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 `svm` or `vmx` flag, but also the `pcid` flag to its guests:

      [libvirt]
      cpu_mode = custom
      cpu_models = IvyBridge
      cpu_model_extra_flags = vmx,pcid

More information on CPU models can be found in CPU models.

### 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.

## KVM performance tweaks

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

    # modprobe vhost_net

## Troubleshooting

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

this page last updated: 2026-08-20 01:33:50

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