Some people prefer to manage their hypervisors entirely via the libvirt control plane. At the moment these people are forced to choose the KVM hypervisor, even though libvirt also supports the Xen hypervisor. This design extends CloudStack to support management of hosts running Xen via libvirt as well as KVM. No change will be made to the XenServer plugin.

Bug Reference



Work in progress (ugly) patches are here:


  1. Maximise code shared between KVM and Xen. Corollary: minimise the number of Xen-specific workarounds
    1. We want people to be able to add features to the libvirt plugin without having to write unnecessary special-cases for Xen or KVM.
    2. If a libvirt hypervisor driver lacks a useful feature, we should implement it.
    3. Share the same system VM template as KVM


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Document History

  • 2014-06-08: initial draft


Feature Specifications

Summary: This design extends the existing KVM/libvirt plugin to be able to manage Xen/libvirt hosts.


  • L1: It will not be possible to support clusters of mixed KVM/Xen hosts. This is because it is not possible to live migrate VMs between different hypervisors.
  • L2: It will not be possible to support clusters of mixed Xen/XenServer hosts.
  • L3: It will not be possible to automatically convert a XenServer cluster into a Xen cluster or vice-versa

Proposed changes

  • C1: The HypervisorType enum will be extended to include "XEN" (alongside "KVM" and "XENSERVER")
  • C2: Create a "XenLibvirtDiscoverer" as a subclass of LibvirtServerDiscoverer (alongside the KVM and LXC versions).
  • C3: Generalise the cloudutils python function "isKVMEnabled" to "isHypervisorEnabled"
  • C3.1 Detect the Xen hypervisor by reading /sys/hypervisor/type
  • C3.2 Detect the KVM hypervisor by "lsmod | grep kvm"
  • C4. Register a system VM template for the "XEN" hypervisor
  • C4.1 Use the same template image as KVM (including keeping it in .qcow2 format)
  • C5. Use the "xen" PV driver family if using "XEN"; and use "virtio" drivers if using "KVM"
  • C6. Under Xen, by default boot the system VM in PV mode
  • C6.1 Add a config setting to enable system VMs to boot in HVM mode
  • C7 In cloud-early-config under Xen, first check the host private "channel" and fall back to the command-line parsing

Dependencies on other projects

This work will depend on changes and improvements in other open-source projects:

  • D1 Xen's libxl needs to support a bidirectional host <-> guest private channel
  • D2 libvirt's libxl driver needs to map <channel> in the domain xml to libxl constructs

Todo: document minimum Xen, libvirt versions


  • The libxl driver in libvirt is less mature than the KVM driver. This might manifest as quality issues or missing features

Use cases

A cloud admin wishes to manage hosts with the libvirt control-plane (e.g. virt-manager, virsh etc) using the Xen hypervisor. The admin installs Xen and libvirt via their favourite Linux distribution. The admin logs into the CloudStack UI and creates a cluster with hypervisor type "XEN". The admin then adds a host to the cluster. A user is requests an instance and a VM is spawned on the Xen host via libvirt.

Architecture and Design description

On the choice of system VM image format: there is no need to choose a different format for Xen vs KVM as they are both able to use the same disk drivers via qemu. Therefore the Xen system VM image format should be set to .qcow2 to minimise divergence.

On using the host <-> guest private <channel> for configuration: this mechanism is used by KVM today and is a sensible design choice and a real missing feature in the libvirt libxl driver. Therefore we should implement the missing feature.

On extending the HypervisorType enum: this is a bit ugly because conceptually we have a single "libvirt" type with 2 subtypes for "Xen" and "KVM". All the code which currently says "if kvm`' needs to become "if kvm || xen".

On detecting hypervisors on the host: it would be better to always use /sys/hypervisor/type, but this doesn't appear to be populated by the KVM module. Another option is to query "virsh capabilities" but this requires the libvirt daemon to be already started, which is not required today.

Web Services APIs

No new APIs are needed

UI flow

No change to the UI flow is needed

IP Clearance

No new dependencies need to be added to CloudStack

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