This website is the archived site of the Xen Project. For up-to-date content, please go to!
Xen Project 
Home Products Support Community Blog

Xen 4.2 Releases

Cambridge, UK 8/9/2012 -, home of the open source Xen hypervisor, today announced the availability of Xen 4.2, the most advanced open source hypervisor software available. The release is the collective effort of a global development team representing many leading technology vendors, universities, and virtualization experts. This release is the culmination of 18 months of work, 2900 commits and almost 300K lines of code of development effort, by 124 individuals from 43 organizations.

The Xen 4.2 release incorporates many new features and improvements to existing features. There are improvements across the board including to Security, Scalability, Performance and Documentation.

  • XL is now the default toolstack: Significant effort has gone in to the XL tool toolstack in this release and it is now feature complete and robust enough that we have made it the default. This toolstack can now replace xend in the majority of deployments, see XL vs Xend Feature Comparison. As well as improving XL the underlying libxl library has been significantly improved and supports the majority of the most common toolstack features. In addition the API has been declared stable which should make it even easier for external toolstack such as libvirt and XCP's xapi to make full use of this functionality in the future.
  • Large Systems: Following on from the improvements made in 4.1 Xen now supports even larger systems, with up to 4095 host CPUs and up to 512 guest CPUs. In addition toolstack feature like the ability to automatically create a CPUPOOL per NUMA node and more intelligent placement of guest VCPUs on NUMA nodes have further improved the Xen experience on large systems. Other new features, such as multiple PCI segment support have also made a positive impact on such systems.
  • Improved security: The XSM/Flask subsystem has seen several enhancements, including improved support for disaggregated systems and a rewritten example policy which is clearer and simpler to modify to suit local requirements.
  • Documentation: The Xen documentation has been much improved, both the in-tree documentation and the Xen 4.2 Wiki Documentations. This is in no small part down to the success of the Xen Document Days so thanks to all who have taken part.

Related Projects

The Xen project continues to work closely with its upstream projects. Of particular note in the Xen 4.2 release cycle is the upstreaming of the HVM device model support into qemu. After the Linux dom0 support (merged upstream in Linux 3.0) the qemu-derived device model was the largest remaining piece of code which required upstreaming. Support for Xen was merged into upstream prior to the qemu 0.15 release and is supported as an option using the XL toolstack. It will become the default in 4.3. Alongside this support we have also gained support for SeaBIOS and Tianocore/OVMF (a UEFI BIOS).


Contributions to Xen 4.2 made to this release by 124 individuals from 43 organizations, not counting contributions to external projects such as the BSDs, Linux or qemu.

The diagram below shows organisations which contributed more than 1% in lines of code to the Xen 4.2 release. Several items in the diagram discribe groups of people or organisations: Individual covers contributions by individuals whose affiliation is unknown, Misc covers contributions by commercial organisations which did not go above 1% individually and University covers contributions by Universities which did not go above 1% individually.

For a complete breakdown of community contributions, see Xen 4.2 Acknowledgements.

Further Information:

About is the home of the open source Xen hypervisor, a fast, secure industry standard code base for operating system virtualization. Founded and led by Ian Pratt, the community benefits from the contributions of senior engineers from leading hardware, software, and security vendors. is run for the benefit of the community by the Xen Project Advisory Board, which is drawn from leading contributors to the project. For more information, visit

Forward-Looking Statements Disclosure

Citrix Systems, Inc. is a contributor to and hosts This release contains forward-looking statements which are made pursuant to the safe harbor provisions of Section 27A of the Securities Act of 1933 and of Section 21E of the Securities Exchange Act of 1934. The forward-looking statements in this release, including without limitation statements made by representatives of contributors to, do not constitute guarantees of future performance. Investors in contributors to are cautioned that statements in this press release, which are not strictly historical statements, involve risks that could cause actual results to differ materially, including risks associated with the impact of the global economy and uncertainty in the IT spending environment, the demand for cloud services, the uncertainty inherent in developing cutting-edge technology, the failure to coordinate and manage contributions to, competition from alternative cloud infrastructure providers, the voluntary nature of contributions to, the level of resources made available to by contributors to as well as other risks detailed in any public statements or filings by or any contributor to Neither nor any contributor to assumes any obligation to update any forward-looking information contained in this press release or with respect to the announcements described herein.


Xen® is a trademark of Citrix Systems, Inc. managed on behalf of, and may be registered in the U.S. Patent and Trademark Office and in other countries. Citrix┬« is a trademark of Citrix Systems, Inc. and/or one or more of its subsidiaries, and may be registered in the U.S. Patent and Trademark Office and in other countries. All other trademarks and registered trademarks are property of their respective owners.