Xen guest ARM64 board¶
This board specification¶
This board is to be run as a virtual Xen  guest with U-boot as its primary bootloader. Xen is a type 1 hypervisor that allows multiple operating systems to run simultaneously on a single physical server. Xen is capable of running virtual machines in both full virtualization and para-virtualization (PV) modes. Xen runs virtual machines, which are called “domains”.
Paravirtualized drivers are a special type of device drivers that are used in a guest system in the Xen domain and perform I/O operations using a special interface provided by the virtualization system and the host system.
Xen support for U-boot is implemented by introducing a new Xen guest ARM64 board and porting essential drivers from MiniOS  as well as some of the work previously done by NXP :
- PV block device frontend driver with XenStore based device enumeration and UCLASS_PVBLOCK class;
- PV serial console device frontend driver;
- Xen hypervisor support with minimal set of the essential headers adapted from the Linux kernel;
- Xen grant table support;
- Xen event channel support in polling mode;
- XenBus support;
- dynamic RAM size as defined in the device tree instead of the statically defined values;
- position-independent pre-relocation code is used as we cannot statically define any start addresses at compile time which is up to Xen to choose at run-time;
- new defconfig introduced: xenguest_arm64_defconfig.
- U-boot runs without MMU enabled at the early stages. According to Xen on ARM ABI (xen/include/public/arch-arm.h): all memory which is shared with other entities in the system (including the hypervisor and other guests) must reside in memory which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable. Thus, page attributes must be equally set for all the entities working with that page. Before MMU is set up the data cache is turned off and pages are seen by the vCPU and Xen in different ways - cacheable by Xen and non-cacheable by vCPU. So it means that manual data cache maintenance is required at the early stages.
- No serial console until MMU is up. Because data cache maintenance is required until the MMU setup the early/debug serial console is not implemented. Therefore, we do not have usual prints like U-boot’s banner etc. until the serial driver is initialized.
- Single RAM bank supported. If a Xen guest is given much memory it is possible that Xen allocates two memory banks for it. The first one is allocated under 4GB address space and in some cases may represent the whole guest’s memory. It is assumed that U-boot most likely won’t require high memory bank for its work andlaunching OS, so it is enough to take the first one.
Board default configuration¶
One can select the configuration as follows:
- make xenguest_arm64_defconfig
 - https://xenproject.org/