Building the Xen Hypervisor with PetaLinux 2017.3

The guide below shows you how to build Xen, boot Xen and then run some example configurations on ZU+. The steps below use PetaLinux and assume you have some knowledge of using PetaLinux.


Table of Contents


Introduction

Before starting you need to create a PetaLinux project. It is assumed that a default PetaLinux reference design is used unchanged in these instructions.
The default PetaLinux configuration has images ready to do boot Xen, these are the pre-built images. You can use those or you can manually edit receipes and build Xen yourself. The pre-built images can be found in this directory (inside a PetaLinux project) pre-built/linux/images/ and prefixed with "xen-". You can either use the pre-builts or follow the next section to configure and build Xen yourself. If you are using the pre-builts you can skip to the booting Xen section for your release version.

Configuring and building XEN from source using PetaLinux

First let's enable Xen to be built by default.
Now let's enable Xen:
Now we need to change the rootFS to be an INITRD
And change
NOTE: This means that any images built will NOT have the rootFS in the Image that is built by PetaLinux. This means you will need to edit any scripts or configuration that expects the rootFS to be included. This includes the Xen configs mentioned later.
You can still use the prebuilt Image file which does still include the rootFS to boot DomU.

We also want to edit the device tree to build in the extra Xen related configs.
Edit this file
and add this line: /include/ "xen-overlay.dtsi".
It should look like this for harware:
or like this for QEMU:
NOTE: There is a bug on QEMU where the CPUs running in SMP sometimes cause hangs. To avoid this we only tell Xen about a single CPU.

Also edit this file:
and add this line to it: file://xen-overlay.dtsi.
The file should look like this:
Then run petaliux-build:

TFTP Booting Xen and Dom0

Run Xen dom0 on QEMU:

To use the prebuilt Xen run:
To use the Xen you built yourself run:

Run Xen dom0 on HW:

To use the prebuilt Xen on hardware:
To use the Xen you built yourself run:
You should eventually see something similar to this, when you do press any key to stop the autoboot.
If u-boot wasn't able to get an IP address from the DHCP server you may need to manually set the serverip (it's typically 10.0.2.2 for QEMU):

Now to download and boot Xen, if running on QEMU, use xen-qemu.dtb otherwise use xen.dtb. Example:

TFTPing Xen from pre-built images

TFTPing Xen from your own images

Below is an example of what you will see.
Login using 'root' as the username and password.

SD Booting Xen and Dom0

To boot Xen from an SD card you need to copy the following files to the boot partition of the SD card:
  1. BOOT.bin
  2. Image
  3. the compiled device tree file renamed to system.dtb (xen.dtb or xen-qemu.dtb for QEMU from the pre-built images, system.dtb from a Petalinux build)
  4. xen.ub
  5. rootfs.cpio.gz.u-boot (Only if using initrd instead of initramfs for the rootfs)

When using the pre-built images from the BSP, copy these files from <project-dir>/pre-built/linux/images. The prebuilt images are built to support both Linux without Xen and with Xen such that some of the Xen based image file names are different than in a normal Petalinux build. The prebuilt Linux kernel image includes an initramfs rootfs. Petalinux builds (rather than prebuilt images) require an initrd rootfs such that another file for the rootfs must also be used as described below.

RootFS in Kernel (initramfs)

This method allows the use of a Linux kernel with an initramfs (such as the prebuilt image). Boot the SD card on hardware or QEMU and stop the u-boot autoboot. At the u-boot prompt run:
This would also allow a rootfs mounted on the SD card to be used. It requires that you extract the rootFS (cpio file) to the root partition on the SD card and setup the kernel to expect the rootfs on the SD card in the kernel command line.

RootFS mounted on RAM (initrd)

This method is required when using a Linux image which is initrd based and does not include a rootfs. The rootfs.cpio.gz.u-boot file will be loaded in memory from u-boot. Then boot the SD card on hardware or QEMU and stop the u-boot autoboot. At the u-boot prompt run:

Starting simple additional guests

If running on QEMU, we'll need to setup a port mapping for port 22 (SSH) in our VM.
In this example, we forward the hosts port 2222 to the VM's port 22.

Once you hit the u-boot prompt, follow the steps in the earlier section on how to run Xen dom0.
When dom0 has finished booting, we'll need to copy a guest Image into dom0's filesystem.
We'll use the base prebuilt PetaLinux Image as our domU guest.

If running on QEMU, we use scp's -P option to connect to our hosts port 2222 where QEMU will forward the connection to the guests port 22:
To target QEMU run the following on the host:
If running on hardware run the following on the host:
If you would prefer to load DomU's kernel to the guest via SD card, you can follow the instructions in the "Starting Linux guests with Pass-through networking" section.

The xen-image-minimal rootFS includes some prepared configurations that you can use. These are located in '/etc/xen/'
To start a simple guest run the following from the dom0 prompt
You'll see another instance of Linux booting up.
At any time you can leave the console of the guest and get back to dom0 by pressing ctrl+].
Once at the dom0 prompt you can list the guests from dom0:
To get back to the guests console:
You can create further guests by for example running:
To destroy a guest:

CPU Pinning

The following will only work on QEMU with multi-core enabled or on real HW.
When running multiple guests with multiple Virtual CPUs, Xen will schedule the various vCPUs onto real physical CPUs.
The rules and considerations taken in scheduling decisions depend on the chosen scheduler and the configuration.
To avoid having multiple vCPUs share a single pCPU, it is possible to pin a vCPU onto a pCPU and to give it exclusive access.

To create a simple guest with one Virtual CPU pinned to Physical CPU #3, you can do the following:

Another way to pin virtual CPUs on to Physical CPUs is to create dedicated cpu-pools.
This has the advantage of isolating the scheduling instances.

By default a single cpu-pool named Pool-0 exists. It contains all the physical cpus.
We'll now create our pool named rt using the credit2 scheduler.
Now we are ready to create a guest with a single vcpu pinned to physical CPU #3.

Starting Linux guests with Para-Virtual networking

This time we will run QEMU slightly different. We'll create two port mappings. One for dom0's SSH port and another for the Para-Virtual domU.
The default IP addresses assigned by QEMUs builtin DHCP server start from 10.0.2.15 and count upwards.
Dom0 will be assigned 10.0.2.15, the next guest 10.0.2.16 and so on.
So here's the command line that maps host port 2222 to dom0 port 22 and 2322 to domUs port 22.

Now, follow the instructions from section 1 on how to boot Xen dom0.
Once you are at the dom0 prompt and have copied a domU image (see earlier steps) we'll need to setup the networking.
In this example, we will configure the guests to directly join the external network by means of a bridge.

First of all, we need to de-configure the default setup.
Kill the dhcp client for eth0:
List and remove existing addresses from eth0:
In our example the address is 10.0.2.15/24:
Then, create the bridge and start DHCP on it for dom0:
You should see something like the following:
Similar to before we will use the pre-defined examples in '/etc/xen/'
The start DomU
You should see a new linux instance boot up.
Now we'll ssh into the domU from the host running Para-Virtual networking:

Starting Linux guests with Pass-through networking

The difficulty with using pass through networking is that the steps above use Dom0 networking to load the DomU boot image onto the guest. This won't work with pass through networking as Dom0 never has any networking avaliable.
You will need to find a way to get the kernel and rootFS (the pre-built Image file) onto the guest. The steps below are used to get the Image file onto a SD card image and attach it to QEMU. Similar steps can be followed for hardware, excpet just copy the Image file to a formated SD card and insert it into the board.
Create and format the file we will be using on your host:
Copy the Image file onto the card.
NOTE: We are using the pre-built Image which contains a kernel and rootFS. If you use the Image you built above then no rootFS is included. You will need to copy the rootFS onto the SD card and edit the Xen config file later to specify a rootFS.
Pass-through does not work in PetaLinux 2017.3 with the example pass-through device tree included in PetaLinux!
You will need to copy this device tree into Dom0 for 2017.3. PetaLinux 2017.4 does not require any changes from the included device tree:
We recommend compiling it to a DTB on the host machine and then copying it onto the SD card:
Now boot QEMU with this extra option appened inside the --qemu-args: "-drive file=qemu_sd.img,if=sd,format=raw,index=1"
The full command should look something like this for your prebuilt images:
The full command should look something like this for your own images:
Then boot Dom0 following the steps above, with one difference. You will need to make sure that you tell Xen about the network passthrough. To do this you will need to edit the device tree. We are going to use u-boot to edit the device tree.
After loading the device tree to memory you will need to run this: fdt addr $fdt_addr && fdt resize 128; fdt set /amba/ethernet@ff0e0000 status "disabled" && fdt set /amba/ethernet@ff0e0000 xen,passthrough "1"
The full command for booting prebuilt images you built is shown below:
The full command for booting images you built is shown below:
NOTE: If running on hardware you will need to make a change to allow the DMA transactions. See here for more details: Passthrough Network Example
Once you have logged onto the system mount the SD card and copy the image.
You also need to replace the included passthrough-example-part.dtb device tree. If it's on the SD card you can run this
Similar to before we will use another pre-defined examples in '/etc/xen/'
Unless you have manually built the latest commit of QEMU from master DomU will have before you reach the login prompt.