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https://github.com/AsahiLinux/u-boot
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81 lines
3.2 KiB
ReStructuredText
81 lines
3.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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Xen guest ARM64 board
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=====================
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This board specification
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------------------------
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This board is to be run as a virtual Xen [1] guest with U-boot as its primary
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bootloader. Xen is a type 1 hypervisor that allows multiple operating systems
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to run simultaneously on a single physical server. Xen is capable of running
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virtual machines in both full virtualization and para-virtualization (PV)
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modes. Xen runs virtual machines, which are called “domains”.
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Paravirtualized drivers are a special type of device drivers that are used in
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a guest system in the Xen domain and perform I/O operations using a special
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interface provided by the virtualization system and the host system.
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Xen support for U-boot is implemented by introducing a new Xen guest ARM64
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board and porting essential drivers from MiniOS [3] as well as some of the work
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previously done by NXP [4]:
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- PV block device frontend driver with XenStore based device enumeration and
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UCLASS_PVBLOCK class;
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- PV serial console device frontend driver;
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- Xen hypervisor support with minimal set of the essential headers adapted from
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the Linux kernel;
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- Xen grant table support;
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- Xen event channel support in polling mode;
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- XenBus support;
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- dynamic RAM size as defined in the device tree instead of the statically
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defined values;
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- position-independent pre-relocation code is used as we cannot statically
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define any start addresses at compile time which is up to Xen to choose at
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run-time;
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- new defconfig introduced: xenguest_arm64_defconfig.
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Board limitations
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-----------------
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1. U-boot runs without MMU enabled at the early stages.
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According to Xen on ARM ABI (xen/include/public/arch-arm.h): all memory
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which is shared with other entities in the system (including the hypervisor
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and other guests) must reside in memory which is mapped as Normal Inner
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Write-Back Outer Write-Back Inner-Shareable.
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Thus, page attributes must be equally set for all the entities working with
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that page.
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Before MMU is set up the data cache is turned off and pages are seen by the
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vCPU and Xen in different ways - cacheable by Xen and non-cacheable by vCPU.
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So it means that manual data cache maintenance is required at the early
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stages.
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2. No serial console until MMU is up.
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Because data cache maintenance is required until the MMU setup the
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early/debug serial console is not implemented. Therefore, we do not have
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usual prints like U-boot’s banner etc. until the serial driver is
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initialized.
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3. Single RAM bank supported.
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If a Xen guest is given much memory it is possible that Xen allocates two
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memory banks for it. The first one is allocated under 4GB address space and
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in some cases may represent the whole guest’s memory. It is assumed that
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U-boot most likely won’t require high memory bank for its work andlaunching
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OS, so it is enough to take the first one.
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Board default configuration
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---------------------------
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One can select the configuration as follows:
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- make xenguest_arm64_defconfig
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[1] - https://xenproject.org/
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[2] - https://wiki.xenproject.org/wiki/Paravirtualization_(PV)
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[3] - https://wiki.xenproject.org/wiki/Mini-OS
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[4] - https://source.codeaurora.org/external/imx/uboot-imx/tree/?h=imx_v2018.03_4.14.98_2.0.0_ga
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