ARMv8 Xen Virtual Machine Example

Overview

This board allows to run Zephyr as Xen guest on any ARMv8 board that supports ARM Virtualization Extensions. This is example configuration, as almost any VM configuration is unique in many aspects.

It provides minimal set of devices:

• ARM Generic timer

• GICv2/GICv3

Hardware

Supported Features

The following hardware features are supported:

Interface	Controller	Driver/Component
GIC	virtualized	interrupt controller
ARM TIMER	virtualized	system clock

The kernel currently does not support other hardware features on this platform.

The default configuration for this board can be found in these files:

• boards/xen/xenvm/Kconfig.defconfig

• boards/xen/xenvm/xenvm_defconfig

Devices

System Clock

This board configuration uses a system clock frequency of 8.32 MHz. This is the default value, which should be corrected for user’s actual hardware.

You can determine clock frequency of your ARM Generic Timer by inspecting Xen boot log:

(XEN) [    0.147541] Generic Timer IRQ: phys=30 hyp=26 virt=27 Freq: 8320 KHz

Interrupt Controller

Depending on the version of the GIC on your hardware, you may choose one of the following board configuration variants:

• xenvm selects GICv2

• xenvm//gicv3 selects GICv3

CPU Core type

Default core in this configuration is Cortex A72. Depending on yours actual hardware you might want to change this option in the same way as Interrupt Controller configuration.

Known Problems or Limitations

Xen configures guests in runtime by providing device tree that describes guest environment. On other hand, Zephyr uses static configuration that should be know at build time. So there are chances, that Zephyr image created with default configuration would not boot on your hardware. In this case you need to update configuration by altering device tree and Kconfig options. This will be covered in detail in next section.

Most of Xen-specific features are not supported at the moment. This includes: * XenBus (under development) * Xen PV drivers

Now only following features are supported: * Xen Enlighten memory page * Xen event channels * Xen PV console (2 versions: regular ring buffer based for DomU and consoleio for Dom0) * Xen early console_io interface (mainly for debug purposes - requires debug version of Xen) * Xen grant tables (granting access for own grants and map/unmap foreign grants)

Building and Running

Use this configuration to run basic Zephyr applications and kernel tests as Xen guest, for example, with the Basic Synchronization sample:

• if your hardware is based on GICv2:

$ west build -b xenvm samples/synchronization

• if your hardware is based on GICv3:

$ west build -b xenvm//gicv3 samples/synchronization

This will build an image with the synchronization sample app. Next, you need to create guest configuration file zephyr.conf. There is example:

kernel="zephyr.bin"
name="zephyr"
vcpus=1
memory=16
gic_version="v2"
on_crash="preserve"

When using xenvm//gicv3 configuration, you need to remove the gic_version parameter or set it to "v3".

You need to upload both zephyr.bin and zephyr.conf to your Dom0 and then you can run Zephyr by issuing

$ xl create zephyr.conf

Next you need to attach to PV console:

$ xl console zephyr

Also this can be performed via single command:

$ xl create -c zephyr.conf

You will see Zephyr output:

*** Booting Zephyr OS build zephyr-v2.4.0-1137-g5803ee1e8183  ***
thread_a: Hello World from cpu 0 on xenvm!
thread_b: Hello World from cpu 0 on xenvm!
thread_a: Hello World from cpu 0 on xenvm!
thread_b: Hello World from cpu 0 on xenvm!
thread_a: Hello World from cpu 0 on xenvm!

Exit xen virtual console by pressing CTRL+]

Updating configuration

As was said earlier, Xen describes hardware using device tree and expects that guest will parse device tree in runtime. On other hand, Zephyr supports only static, build time configuration. While provided configuration should work on almost any ARMv8 host running in aarch64 mode, there is no guarantee, that Xen will not change some values (like RAM base address) in the future.

Also, frequency of system timer is board specific and should be updated when running Zephyr xenvm image on new hardware.

One can make Xen to dump generated DTB by using LIBXL_DEBUG_DUMP_DTB environment variable, like so:

$ LIBXL_DEBUG_DUMP_DTB=domu-libxl.dtb xl create zephyr.conf

Then, generated “domu-libxl.dtb” file can be de-compiled using “dtc” tool.

Use information from de-compiled DTB file to update all related entries in provided “xenvm.dts” file. If memory layout is also changed, you may need to update CONFIG_SRAM_BASE_ADDRESS as well.

References

Xen ARM with Virtualization Extensions

xl.conf (guest configuration file) manual