mirror of
https://github.com/AsahiLinux/u-boot
synced 2024-12-24 20:13:39 +00:00
82 lines
3.2 KiB
ReStructuredText
82 lines
3.2 KiB
ReStructuredText
|
.. 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
|