doc: xen: Add Xen guest ARM64 board documentation

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>

doc/board/index.rst

@@ -22,4 +22,5 @@ Board-specific doc
    st/index
    tbs/index
    toradex/index
+   xen/index
    xilinx/index

doc/board/xen/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+XenGuestARM64
+=============
+
+.. toctree::
+   :maxdepth: 2
+
+   xenguest_arm64

doc/board/xen/xenguest_arm64.rst

@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Xen guest ARM64 board
+=====================
+
+This board specification
+------------------------
+
+This board is to be run as a virtual Xen [1] 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 [3] as well as some of the work
+previously done by NXP [4]:
+
+- 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.
+
+
+Board limitations
+-----------------
+
+1. 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.
+
+2. 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.
+
+3. 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
+
+[1] - https://xenproject.org/
+
+[2] - https://wiki.xenproject.org/wiki/Paravirtualization_(PV)
+
+[3] - https://wiki.xenproject.org/wiki/Mini-OS
+
+[4] - https://source.codeaurora.org/external/imx/uboot-imx/tree/?h=imx_v2018.03_4.14.98_2.0.0_ga