ARMv8 Xen Virtual Machine Example¶
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
SBSA (subset of PL011) UART controller
The following hardware features are supported:
The kernel currently does not support other hardware features on this platform.
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
This board configuration uses a single serial communication channel using SBSA UART. This is a minimal UART implementation provided by Xen. Xen PV Console is not supported at this moment.
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.
No Xen-specific features are supported at the moment. This includes:
Xen Enlighten memory page
Xen event channels
Xen grant tables
Xen PV drivers (including PV console)
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" vuart="sbsa_uart"
You need to upload both
zephyr.conf to your Dom0
and then you can run Zephyr by issuing
$ xl create zephyr.conf
Next you need to attach to SBSA virtual console:
$ xl console -t vuart zephyr
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+]
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
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
CONFIG_SRAM_BASE_ADDRESS as well.