Xen ARM with Virtualization Extensions

From Xen
Revision as of 10:38, 25 February 2013 by Ijc (talk | contribs) (First pass at incorporating v8 information)

The ARM v7-A and ARM v8-A architectures include optional virtualization extensions that allow a hypervisor to manage fully hardware virtualized guests. These extensions are currently available in some ARM v7 processors such as the Cortex A15 and Cortex A7.


Both the 32-bit (arm32) and the 64-bit (arm64) ports of Xen boot dom0 and unprivileged guests can be created and destroyed using xl. See below for information on the hardware and models.

Currently only 32-bit dom0 and guests are supported, even on the 64-bit hypervisor.

Guest ABI

The guest (including dom0) visible ABI exposed by the hypervisor is not yet set in stone and there may be changes as work progresses. See requirements (below) for specific details of matching kernel and hypervisor versions.


Please email xen-devel with comments, questions and patches. Please see the list info page for subscription information and the archives. For patches please see Submitting Xen Patches.


The arm32 port of Xen currently runs on:

Work is also underway to port Xen to the Exynos 5250 processor which is in the Google Chromebook and Arndale Development board.

The arm64 port of Xen currently runs on:

  • The AEMv8 Real-time System Models by ARM.

Work is also underway to support the ARM v8 Foundation Model.


ARM Fast Model
Either the v7-A ARM Envelope Model (AEM) or the Cortex A15/A7 models (single CPU only).
Device Tree
A device tree in the flat device tree format (.dtb). The device tree source for unprivileged Xen ARM guests is available upstream in the Linux tree >= v3.7: xenvm-4.2.dts. The interface is also documented in the Linux tree: xen.txt. Regarding Xen and Dom0, the regular DTS for the underlying platform can be used, however it requires the additional hypervisor node described in the Linux Documentation and the command line arguments under the chosen node, see for example the Xen DTS for the Versatile Express Cortex A15 machine we use for development vexpress-v2p-ca15-tc1.dts.
All current work is now merged into the current development branch [1] or [2]. It is recommended to use the latest xen-unstable commit.
Linux kernel for dom0
The patches necessary to boot Linux as dom0 under Xen were merged upstream in v3.7. However in order to actually start guests a few additional patches are required. These patches can be found in the arm-privcmd-for-3.8 branch of git://xenbits.xen.org/people/ianc/linux.git and they went upstream in Linux v3.8-rc1. The latest Linus' tree has everything needed to run on Xen on ARM as dom0 and domU.
dom0 userspace
The developers are using the armhf port of Debian Wheezy (which is currently the testing release).
domU kernel
The patches necessary to boot Linux as a guest under Xen were merged upstream in v3.7.

ABI Compatibility

The ABI for Xen on ARM is currently not set in stone. The branches and trees referenced above are self consistent.

Device Trees

Xen needs the device trees to be in the flat device tree format (the device tree blob or DTB). This is built by the device tree compiler (dtc) from the device tree source files (.dts and .dtsi).

Version 1.3 or later of dtc is required.

Build the .dtb files with make.

Pre-built vexpress-v2p-ca15-tc1.dtb and xenvm-4.2.dtb are available.


When running on the model there is no bootloader to provide the DTB. Instead, it is linked with the xen image.

The DTB to use is specified by the CONFIG_DTB_FILE config variable which should be set to the absolute path of the DTB. This variable may be set in .config or on the make command line.

The Xen command line may be set with the bootargs parameter of the chosen node of the device tree.

Dom0 kernel

Enable at least ARCH_VEXPRESS, ARCH_VEXPRESS_DT. Disable SPARSE_IRQ (it doesn't play nicely with device tree support). If ARM_APPENDED_DTB is enabled then any appended DTB will be used instead of one supplied by Xen and the kernel will crash unless the memory in the DTB matches that location/size supplied by Xen.

A working configuration is available here.

The Linux command line should be set with the xen,dom0-bootargs parameter of the chosen node of the device tree supplied to Xen. If this parameter is missing then Linux will not get a command line and the wrong console will be used (there's a bug where the kernel doesn't use the default, built-in command line if the /chosen node is missing.

DomU kernel and DTS

Unprivileged guests can be created using xl. A simple VM config file would look like this:

kernel = "/root/kernel"
memory = 128
name = "win"
vcpus = 1
disk = [ 'phy:/dev/loop0,xvda,w' ]

where the kernel is actually a Linux zImage with an appended xenvm-4.2.dtb.

Flash Image

The dom0 kernel is loaded by Xen from the beginning of the flash.

Native boot

To boot the kernel natively use an updated boot-wrapper from git://xenbits.xen.org/people/dvrabel/boot-wrapper.git.

Build the device tree blobs provided with the kernel with make dtbs. A suitable DTB file for the envelope model is vexpress-v2p-aem-v7a.dtb.

For a native boot the kernel should have ARM_APPENDED_DTB and ARM_ATAG_DTB_COMPAT enabled. The kernel zImage and the DTB should be appended. e.g., cat zImage vexpress-v2p-aem-v7a.dtb > image.bin.

Building Xen on ARM

Cross Compiling

Cross compiling the hypervisor is relatively simple. You can find a cross compiler in some Linux distros or you can download the arm-unknown-linux-gnueabi compiler from kernel.org.

Once you have a suitable cross compiler you can compile Xen with:

   $ make dist-xen XEN_TARGET_ARCH=arm32 CROSS_COMPILE=arm-unknown-linux-gnueabi-

This assumes that the command prefix for you cross compiler is arm-unknown-linux-gnueabi- and that arm-unknown-linux-gnueabi-gcc and friends are in your $PATH.

Unfortunately cross-compiling the tools is not currently supported

Native Building

In order to build the tools a native build environment is required. The developers mainly use the armhf port of Debian, which is present in Wheezy (the current testing distribution) running on an IMX53 based development board, although any ARMv7 development board would do. Note that the build hardware does not need to support the virtualisation extensions, since you don't have to run Xen on the same system as where you build it.

It may also be possible to build using a cross-architecture chroot running on an X86 system as described in this blog post, although this hasn't been tried yet.

Versatile Express specific configurations

  • If you are trying to run Xen on a Versatile Express Cortex A15 machine, you might want to copy xen.bin (the Xen binary output) and the Linux Dom0 zImage to the SOFTWARE directory and add the two following stanzas to your images.txt file:
NOR3ADDRESS: 0c000000               ;Image Flash Address
NOR3FILE: \SOFTWARE\Xen\xen.bin     ;Image File Name
NOR3LOAD: 80200000                  ;Image Load Address
NOR3ENTRY: 80200000                 ;Image Entry Point

NOR4ADDRESS: 000c0000               ;Image Flash Address
NOR4FILE: \SOFTWARE\Xen\zImage      ;Image File Name
NOR4LOAD: 80008000                  ;Image Load Address
NOR4ENTRY: 80008000                 ;Image Entry Point
  • We recommend disabling the A7 cpus by editing the board.txt file under the SITE directory corresponding to your Cortex A15 daughterboard:
;SCC: 0x018 0x1FFFFFFF           ;Reset control (CA7s running, CA15s running) - uncomment this for normal operation
;SCC: 0x018 0x1FFFF000           ;Reset control - (CA7s running, CA15s reset) - uncomment this to hold A15 cluster in reset
SCC: 0x018 0x00001FFF            ;Reset control - (CA7s reset, CA15s running) - uncomment this to hold A7 cluster in 
  • The address of the Dom0 kernel in Flash at the moment is not configurable, it is defined as KERNEL_FLASH_ADDRESS in Xen. On a Versatile Express machine this address can be determined by executing flash list images from the boot manager. You'll have to manually change it.
  • In order to make your Dom0 boot sequence faster, you might want to consider passing init=/bin/bash to your kernel and then manually executing a minimal init script like this.

Known Problems and Workarounds

  • SMP support is under development in Xen ARM as we speak, we recommend exporting only 1 cpu in the DTS for the moment.
  • Linux v3.8-rc2 fails to change the MMC voltage during boot on Versatile Express, a patch is needed to fix the bug.

Developer FAQs

Use the Xen ARMv7 Dev FAQ page to cover commonly asked questions.

Related pages