Category Archives: Releases

Information about a new release of software

Announcing the Windows PV Console Driver

It has long been the case that all HVM guests under Xen are provided with a PV console. You can attach to this console in the same way that you attach to the console of a PV guest, by typing in the control domain:

xl console name_of_guest

Until recently there has been no Windows PV driver interaction with this console. Starting with this commit support for logging via the PV console was added to the XENBUS driver.

I’m happy to announce that the three commits to XENBUS starting with this one added the necessary infrastructure to support a brand new XENCONS PV driver which exposes the PV console to Windows user-space as a character device.

The XENCONS driver source is hosted alongside the other PV driver sources on and development builds are available for download here.

The XENCONS package also contains a Windows service to monitor the presence of the PV console device and invoke a command shell login process with redirected stdin/stdout. This means that, once the driver package has been installed, if you attach to the PV console and hit ENTER you’ll see a prompt something like this:


From this prompt you can log in as any local user and you’ll then be presented with the command shell:

Microsoft Windows [Version 10.0.15063]
(c) 2017 Microsoft Corporation. All rights reserved.


Be aware that this shell is running in session 0 so does not have access to the interactive session, but you can still use it for many administrative tasks. For instance, you can run netsh to display aspects of your network configuration:

netsh>interface ipv4 show global
Querying active state...

General Global Parameters
Default Hop Limit : 128 hops
Neighbor Cache Limit : 256 entries per interface
Route Cache Limit : 4096 entries per compartment
Reassembly Limit : 33420160 bytes
ICMP Redirects : enabled
Source Routing Behavior : dontforward
Task Offload : enabled
Dhcp Media Sense : enabled
Media Sense Logging : disabled
MLD Level : all
MLD Version : version3
Multicast Forwarding : disabled
Group Forwarded Fragments : disabled
Randomize Identifiers : enabled
Address Mask Reply : disabled
Minimum Mtu : 576
Locality Address Selection : disabled
Flow Label : disabled

Current Global Statistics
Number of Compartments : 1
Number of NL clients : 7
Number of FL providers : 4

Over the coming weeks I intend to add to the functionality that the driver provides. One obvious extension would be some form of hotkey support to link into the XENBUS_DEBUG interface to enable PV drivers to register a callback to be triggered by a particuler key.

If you are interested in this then please try the XENCONS package and send feedback to the mailing list.

Download server change for Xen releases

The official way to get the Xen hypervisor and other Xen Project downloads is via the the website. If you get Xen via the links on the website, you do not need to read the rest of this message.

We are aware that some users have been visiting the download server directly. That download server is changing.

In the past, the Xen Project has hosted its releases on space kindly provided on by Citrix (and, previously, XenSource). For some time now, we have in parallel made available downloads on the Xen Project’s server at

Starting right away, Xen Project releases will appear only on the Xen Project’s server.

The directory structure remains unchanged. So, you can replace
at the start of all urls, with
in all scripts, bookmarks, etc.

Previously published files will remain on, but new releases will not appear there.

Xen Project 4.8.1 is available

I am pleased to announce the release of Xen 4.8.1. Xen Project Maintenance releases are released in line with our Maintenance Release Policy. We recommend that all users of the 4.8 stable series update to the latest point release.

The release is available from its git repository;a=shortlog;h=refs/heads/stable-4.8
(tag RELEASE-4.8.1) or from the XenProject download page

These releases contain many bug fixes and improvements. For a complete list of changes, please check the lists of changes on the download pages.

Xen Project Maintenance Releases Available (Versions 4.6.5 and 4.7.2)

I am pleased to announce the release of Xen 4.6.5 and 4.7.2. Xen Project Maintenance releases are released in line with our Maintenance Release Policy. We recommend that all users of the 4.6 and 4.7 stable series update to the latest point release.

Xen 4.6.5

Xen 4.6.5 is available immediately from its git repository;a=shortlog;h=refs/heads/stable-4.6
(tag RELEASE-4.6.5) or from the Xen Project download page

Xen 4.7.2

Xen 4.7.2 is available immediately from its git repository;a=shortlog;h=refs/heads/stable-4.7
(tag RELEASE-4.7.2) or from the Xen Project download page

These releases contain many bug fixes and improvements. For a complete list of changes, please check the lists of changes on the download pages.

Xen Project 4.7 and 4.6.3 Release

I’m pleased to announce the release of Xen Project Hypervisor 4.7 and Xen Project Hypervisor 4.6.3.

Xen Project Hypervisor 4.7

This new release focuses on improving code quality, security hardening, security features, live migration support, usability improvements and support for new hardware features — this is also the first release of our fixed term June – December release cycle.

We continue to strive to make Xen Project Hypervisor the most secure open source hypervisor to match the security challenges in cloud computing, and for embedded and IoT use-cases. We are continuing to improve upon the performance and scalability for our users, and aim to continuously bring many new features to our users in a timely manner.

To make it easier to understand the major changes during this release cycle, I’ve grouped them below into several categories:

  • Security Features
  • Migration Support
  • Performance and Workloads
  • Support for new Hardware Features
  • Drivers and Devices (Linux, FreeBSD and other)

Security Features

Reboot-free Live Patching: Xen Project Hypervisor 4.7 comes equipped with Live Patching, a technology that enables re-boot free deployment of security patches to minimize disruption and downtime during security upgrades for system administrators and DevOps practitioners. Xen Project 4.7 implements version 1 of the Xen Project’s Live Patching specification, which is designed to encode the vast majority of security patches (approximately 90%) as Live Patching payloads. This version ships with a Live Patching enabled hypervisor and payload deployment tools and is available as a technology preview.

KCONFIG support: For security, embedded automotive and IoT use cases, Xen Project introduced the ability to remove core Xen Hypervisor features at compile time via KCONFIG. This ability creates a more lightweight hypervisor and eliminates extra attack surfaces that are beneficial in security-first environments, microservice architectures and environments that have heavy compliance and certification needs, like automotive.

Improvements to the Virtual Machine Introspection (VMI) subsystem: A number of performance, scalability, robustness and interface improvements have been added to the Virtual Machine Introspection subsystem, that was introduced in Xen 4.5. In addition, Bitdefender Hypervisor Introspection leveraging Xen Project Virtual Machine Introspection, has recently been released as a new enterprise security solution to discover and remedy deep threats that remain hidden via traditional endpoint security tools.

Foundation work to tolerate a restartable Dom0: Several key components in a Xen Project system run in Dom0, which make Dom0 the single point of failure. Xen Project has been able to run xenstored, the daemon for managing the hypervisor’s central settings repository on a Xen Project host, in a sandboxed Virtual Machine called xenstored stub domain since Xen Project version 4.2. In Xen 4.7, we have made it easier to build xenstored stub domains and for them to tolerate a Dom0 restart. This will make Dom0 less critical to a Xen Project system and help us move towards a more robust and secure architecture in the future. More work in this area is expected in subsequent releases.

Migration Support

Improved Migration support: CPU ID Levelling enables migration of VM’s between a larger range of non-identical hosts than previously supported.

Fault Tolerance / Coarse-grained Lock-stepping (COLO): Xen 4.5 laid the foundation for COLO while improving the Xen Project’s Hypervisors Live Migration and Remus High Availability support. The COLO Manager, which introduces a relaxed approach to checkpointing that avoids unnecessary checkpoints enabling near native performance for many workloads, has been fully integrated as an experimental feature into Xen 4.7. Note that the COLO Block Replication and COLO Proxy components, both of which are QEMU components, are currently still reviewed by the QEMU community. Both components are available as out-of-tree add-ons to the Xen Project Hypervisor, until fully integrated into QEMU.

Performance and Workloads

Support for a wider range of workloads and applications: The PV guest limit restriction of 512GB has been removed to allow the creation of huge PV domains in the TB range. TB sized VMs, coupled with Xen Project’s existing support for 512 vCPUs per VM, enable execution of memory and compute intensive workloads such as big data analytics workloads and in-memory databases.

Improved Credit 2 scheduler: The Credit2 scheduler is one (big) step closer to being ready for production use. It is now possible to instruct the scheduler to organize its runqueues and perform load balancing at core, socket or NUMA node granularity. More fine grained (core) configurations, deliver more aggressive load balancing, and are best suited for medium size systems. This feature has been proven to enable very good performance, especially if Hyper Threading is present.

Less fine grained configurations entail less overhead, and is suitable for larger servers or when no Hyper Threading is available. In addition, Credit2 has been extended to allow pinning of vCPUs to pCPUs (also known as “hard affinity”), allowing system administrators to configure the system in the exact way they want, and achieve the best setup for a given workload (for instance, a guarantee that a certain subset of vCPUs are always able to run when they need to run).

Improved RTDS scheduler: The RTDS scheduler is a real-time CPU scheduler built to provide guaranteed CPU capacity to guest VMs on SMP hosts, which primarily targets embedded, real-time and low-latency workloads. In Xen Project 4.7, the scheduling model has been changed from a quantum-driven to an event-driven model, which reduces scheduling overhead and thus scalability and performance for embedded and realtime workloads. In addition, per-VCPU parameter configuration has been added to allow better scheduler control for specialised workloads.

Per-cpu reader-writer lock: This new infrastructure allows for the fast path read case to have low overhead by only setting/clearing a per-cpu variable for using the read lock. After transforming various hypervisor locks to this infrastructure, VM-VM network transfer with 16 queues jumped from 15 gbit/s to 48 gbit/s on a 2 socket Haswell-EP host.

Usability Improvements

PVUSB Support: In Xen Project 4.7, a new XL command line interface to manage PVUSB devices has been introduced to manage PVUSB devices for PV guests. Both in kernel PVUSB backend and QEMU backend are supported.

Hot plugging of QEMU disk backends: Xen Project now enables hot-plugging of USB devices as well as QEMU disk backends, such as drbd, iscsi, and more in HVM guests. This new feature allows users to add and remove disk backends to virtual machines without the need to reboot the guest.

Soft-reset: The soft reset feature for HVM guests allows for a more graceful shutdown and restart of the HVM guest.

New Hardware Support

Features specific to the ARM Architecture

SBBR Compliance: Xen Project now supports booting on hosts that expose ACPI 6.0 (and later) information. The ARM Server Base Boot Requirements (SBBR) stipulate that compliant systems need to express hardware resources with ACPI; thus this support will come in useful for ARM Servers. This effort was carried out by Shannon Zhao of Linaro with minor patches from Julien Grall of ARM.

PCSI 1.0 Compatibility: PSCI 1.0 compatibility allows Xen Project software to operate on systems that expose PSCI 1.0 methods. Now, all 1.x versions of PSCI will be compatible with Xen Project software. More information on Power State Coordination Interface can be found here. This effort was also carried out by Julien Grall with a patch from Dirk Behme of Bosch.

vGIC-v3: Virtual Generic Interrupt Controller version 3. Reworked to be spec-compliant and optimised in some code paths.

Wallclock support: ARM guest can now get wallclock time directly from Xen Project via shared info page.

Features specific to Intel® Xeon® processor product family

Improved Interrupt Efficiency: Xen Project 4.7 supports VT-d Posted Interrupts, which provides hardware-level acceleration to increase interrupt virtualization efficiency. It reduces latency and improves user experience through performance improvements, especially for interrupt-intensive front-end workloads such as web servers. Note that Posted Interrupts in Xen Project 4.7 are still experimental and disabled by default.

Code and Data Prioritization: Xen Project 4.7 is the first to include Code and Data Prioritization (CDP), part of the Intel® Resource Director Technology (RDT) Framework and an extension of Cache Allocation Technology (CAT), first introduced in Xen Project 4.6. The introduction of CDP allows isolation of code/data within the shared L3 cache of multi-tenant environments, reducing contention and improving performance.

Other Intel Features: Additional features specific to the Intel Xeon processor family in Xen Project 4.7 include: VMX TSC Scaling, which allows for easier migration between machines with different CPU frequencies and support for Memory Protection Keys, a new security feature for hardening the software stack.

Drivers and Devices (Linux, FreeBSD and other)

During the Xen Project 4.7 release cycle, we made significant improvements to major operating systems and components we rely on to improve interoperability. During this development cycle 1494 Xen Project only related changesets – mostly bug fixes and small improvements – were applied to Linux, FreeBSD, NetBSD, QEMU and the Windows PV drivers: more than twice as many as in the 4.6 release cycle.


With dozens of major improvements, many more bug fixes and small improvements, and significant improvements to Drivers and Devices, Xen Project 4.7 reflects a thriving community around the Xen Project Hypervisor.

We are extremely proud of achieving the highest quality of the release while increasing development velocity across the hypervisor and its upstream dependencies by about 16%. In particular, our latest security related features enable Xen Project software to compete in the security appliance market and help answer some of the difficult questions regarding security in the cloud era.

We set out at the beginning of this release cycle to foster greater collaboration among vendors, individual developers, upstream maintainers, other projects and distributions. During this release cycle we continued to see an increasing influx of patches and newcomers such as Star Lab, Bosch and Netflix. We had a significant amount of contributions from cloud providers, software vendors, hardware vendors, academic researchers and individuals to help with this release. Major contributors for this particular release come from Citrix, SUSE, Intel, Star Lab, Oracle, Linaro, Fujitsu, Bitdefender, Red Hat, Huawei, ARM, Novetta, Broadcom, Xilinx, Bosch, AMD, GlobalLogic, NSA, Netflix and a number of universities and individuals. Thank you to all who participated.

As the release manager, I would like to thank everyone for their contributions (either in the form of patches, bug reports or packaging efforts) to the Xen Project. This release wouldn’t have happened without contributions from so many people around the world. Please check out our 4.7 contributor acknowledgement page.

The source can be located in the;a=shortlog;h=refs/heads/stable-4.7 tree (tag RELEASE-4.7.0) or can be downloaded as tarball from our website. More information can be found at

Xen Project Hypervisor 4.6.3

The Xen Project 4.6.3 release is a maintenance release which comprises bug fixes and security updates. This is release is available immediately from its git repository;a=shortlog;h=refs/heads/stable-4.6
(tag RELEASE-4.6.3) or from the Xen Project download page
(where a list of changes can also be found).

We recommend all users of the 4.6 stable series which do not wish to upgrade to Xen 4.7, to update to this latest point release.

Note regarding version numbering: an issue was found late in the release process,
after one of the affected qemu trees was already tagged with a signed release git tag. Signed git tags provide a secure way of accounting for the source code, but once created they cannot be removed. Thus, the project could have released this maintenance release with a known issue, or fix the issue and skip a version number. We opted for the latter and decided to skip version 4.6.2.

Xen Project 4.7 Planning Opens

With Xen 4.6 released in October, we are already one month into the new cycle. Which means it is time to start planning for the next release. You may remember that one of the goals of the 4.6 release planning was to create smoother developer experience and to release Xen 4.6 on time. Both goals were achieved, so it was time to think where to go from here. Thus, the Xen community underwent a thorough discussion on how to manage future releases from xen-unstable and its impact on stable releases. The takeaway message of those lengthy threads is that we should continue to work on making the release cycle shorter and more predictable.

As such, the timeline for 4.7 is:

  • Development starts: October 13, 2015
  • Last posting date: March 18, 2016
  • Hard code freeze: April 1, 2016
  • Release date: June 3, 2016

After the 4.7 release, we will start to release Xen every 6 months: at the beginning of June and December. A regular 6 monthly release schedule has worked well for Ubuntu, OpenStack and many other projects. The idea behind it is a simple one: set a hard date and modify your goals to match that timeline. Which is also, why we dropped feature freeze exceptions, which create overheads and introduce unnecessary risk and debate. In addition, the new fixed release schedule will help open source projects and commercial vendors who consume Xen to plan their own releases better. And it allows us to set a schedule that ensures that every single release cycle is only affected by a single holiday period and that we have a Xen Project developer event (be it a Hackathon or Xen Project Developer Summit) during each release cycle. The stable release scheme is unchanged: 18 months full support, plus 18 months security fixes afterwards.

For more information, check out the slides that explain our release process and how it is changing for Xen 4.7 and beyond. To follow the roadmap in the coming months, be sure to check the Xen 4.7 Roadmap page on our wiki. Get involved on xen-devel@ and happy hacking!

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