Xen Project 4.4 Release Notes

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Overview

Xen Project 4.4.0 was released on March 10, 2014.

Xen Project 4.4 is the work of 8 months of development, with 1193 changesets.

Xen Project 4.4 is our first release made with an attempt at a 6-month development cycle. Between Christmas, and a few important blockers, we missed that by about 6 weeks; but still not too bad overall.

New Feature Highlights

Probably one of the most important is solid libvirt support for libxl. Jim Fehlig from SuSE and Ian Jackson from Citrix worked together to test and improve the interface between libvirt and libxl, making it fast and reliable. This lays the foundation for solid integration into any tools that can use libvirt, from GUI VM managers to cloud orchestration layers like CloudStack or OpenStack.

Another big one is a new scalable event channel interface, designed and implemented by David Vrabel from Citrix. The original Xen event channel interface was limited to the number of bits on the platform squared -- 1024 for 32-bit guests and 4096 for 64-bit guests. With many VMs requiring 4 event channels each, that means a theoretical maximum of 256 guests on a 32-bit dom0 -- more than enough back when a large machine had 8 cores, and every VM was a full OS; but a major limitation on systems with 128 cores, or those using cloud OSes like Mirage or OSv. The new "FIFO" event channel interface by default scales up to over 100,000 event channels, and in the future can be extended even further if necessary in a backwards-compatible manner. This should be enough for many years to come.

The ARM port is maturing quickly. As of 4.4, the hypervisor ABI for ARM has been declared stable, meaning that any guest which uses the 4.4 ARM ABI can rely on being able to boot on all future versions of Xen. There are a number of improvements making Xen on ARM more flexible, easier to set up and use, and easier to extend to new platforms.

There are many more improvements and changes under the hood. For a more complete list, see the Xen 4.4 Feature List.

Build Requirements

The default installation prefix has been changed to /usr/local. Previously the default installation prefix was /usr To restore the previous behaviour, specify --prefix=/usr when running ./configure, e.g.: 

$ ./configure --prefix=/usr

If you choose to accept the new default then be sure to cleanup any residual bits of the old installation in the old prefix, especially libraries and tools which you might pick up by mistake.

Due to this change it may also now be necessary to run /sbin/ldconfig as root after installing Xen.

Note also that Xen now requires gcc v4.1 or greater.

Other changes

  • Support for x86 32-bit and ia64 (itanium) hypervisors has been removed. 32bit VMs, including 32bit dom0, are still supported on the x86_64 64bit hypervisor.

Known issues

  • The Xen Security Module (XSM) will not compile on the ARM architecture.
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