Xen Project 4.21 Release Notes
Overview
Xen Project 4.21.0 was released on 19th of November 2025.
Xen Project 4.21.0 is the work of 8 months of development.
New Feature Highlights
The list of new features can be found on the following page: Xen Project 4.21 Feature List
You can find information on support status (including security support) for various features in SUPPORT.md and Hypervisor Support Matrix.
Documentation is available online: Xen 4.21 Documentation
Build Requirements
Xen requires gcc v5.1 or later (x86), gcc 5.1 or later (arm32), or gcc 5.1 or later (arm64), gcc 12.2 or later (riscv64).
Build
First of all you will have to get the Xen 4.21 source code, which you can either download through our release tarball (including signature) from our download page. Alternatively, you can get the source in the following way:
- With a recent enough git (>= 1.7.8.2) just clone and checkout the proper tag (RELEASE-4.21.0) from the xen.git repo directly:
git clone -b RELEASE-4.21.0 https://xenbits.xenproject.org/git-http/xen.git
Build instructions can be found at Compiling Xen From Source or Compiling Xen From Source on NetBSD.
Changes affecting downstreams and users
- The minimum toolchain requirements have increased for some architectures
- New dependency on library json-c 0.15 or later, the toolstack will prefer it to `YAJL` when available.
- Introduce libxenmanage as a stable library, replacing xenstored's dependency on unstable libraries.
- Allow configuring the number of altp2m tables per domain via vm.cfg.
- Introduce new PDX compression algorithm to cope with Intel Sierra Forest and Granite Rapids having sparse memory maps.
- New amd-cppc/amd-cppc-epp cpufreq driver.
- Remove GNTTABOP_cache_flush: it's unused on x86 and the implementation is broken.
- Support of qemu-traditional has been removed.
- The in-tree oxenstored is deprecated and will be removed in a future version of Xen. It is moving into the Xapi project https://github.com/xapi-project/oxenstored so it can be maintained in line with the other Ocaml projects in the Xen ecosystem.
Known issues
While we do the utmost to make the release perfect, there are always fixes which are not quite complete on time. We will add those here. So if you run into trouble, please check this section.
Currently known issues are:
- xen: Strip xen.efi by default ( https://lore.kernel.org/xen-devel/8f8a769d-95ea-4554-8ee0-d6247f583e37@citrix.com/ )
