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.
Fig. 178 Xen virtual Guest (Credit: Xen Project)
It provides minimal set of devices:
ARM Generic timer
GICv2
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.
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
By default, GICv2 is selected. If your hardware is based on GICv3, you can configure Zephyr to use it, by amending device tree and Kconfig option in “xenvm” SoC as well as guest configuration file.
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 grant tables (under development) * Xen PV drivers
Now only following features are supported: * Xen Enlighten memory page * Xen event channels (currently only pre-defined - PV console, Xenbus) * 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)
Building and Running
Use this configuration to run basic Zephyr applications and kernel tests as Xen guest, for example, with the Synchronization Sample:
$ west build -b xenvm 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"
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.