Xen

This document explains how to use Xen 4.2 in Arch. It uses the new oxenstored / xl toolstack (replaces the xend / xm toolstack which was deprecated in Xen 4.1).

What is Xen?

According to the Xen development team:

"The Xen hypervisor, the powerful open source industry standard for virtualization, offers a powerful, efficient, and secure feature set for virtualization of x86, x86_64, IA64, PowerPC, and other CPU architectures. It supports a wide range of guest operating systems including Windows®, Linux®, Solaris®, and various versions of the BSD operating systems."

The Xen hypervisor is a thin layer of software which emulates a computer architecture. It is started by the boot loader of the computer it is installed on, and allows multiple operating systems to run simultaneously on top of it. Once the Xen hypervisor is loaded, it starts the "Dom0" (short for "domain 0"), or privileged domain, which in our case runs a Linux kernel (other possible Dom0 operating systems are NetBSD and OpenSolaris). The physical hardware must, of course, be supported by this kernel to run Xen. Once the Dom0 has started, one or more "DomUs" (short for user domains, sometimes called VMs) can be started and controlled from Dom0.

Xen.org provides a full overview

Types of Virtualization Available with Xen

Paravirtual (PV)

Paravirtualized guests require a kernel with support for Xen built in. This is default for all recent Linux kernels and some other Unix-like systems. Paravirtualized domUs usually run faster than HVM domains as they do not have to run in emulated hardware.

Hardware Virtual (HVM)

For OSes that do not natively support Xen (e.g. Windows), HVM offers full hardware virtualization. To use HVM in Xen, the host system hardware must include either Intel VT-x or AMD-V (SVM) virtualization support. In order to verify this, run the following command on the host system:

grep -E "(vmx|svm)" --color=always /proc/cpuinfo

If the above command does not produce output, then hardware virtualization support is unavailable and your hardware is unable to run Xen HVM guests. It is also possible that the host CPU supports one of these features, but that the functionality is disabled by default in the system BIOS. To verify this, access the host system's BIOS configuration menu during the boot process and look for an option related to virtualization support. If such an option exists and is disabled, then enable it, boot the system and repeat the above command.

Obtaining Xen

Xen is available from the AUR. The recommended current stable version is Xen 4.2, and the bleeding edge unstable package can be found here. Both packages provide the Xen hypervisor, current xl interface and all configuration and support files, including systemd services.

Xen, unlike certain other virtualization systems, relies on a full install of the base operating system. Before attempting to install Xen, your host machine should have a fully operational and up-to-date install of Arch Linux. If you are building a new host from scratch, see the Installation Guide for instructions on installing Arch Linux.

Like all AUR packages, the Xen binaries are built from source. Note that it is possible (but not necessary) to build the package on a separate machine and transfer the xz package over, assuming that the machines share the same architecture (e.g. x86_64). For Xen, an internet connection is needed during its compilation because further source files are downloaded during the process. Xen.org recommends a host to be 64-bit. This requires the 'multilib' repository to be enabled in etc/pacman.conf.

To build the package you will need the following:

base-devel zlib lzo2 python2 ncurses openssl libx11 yajl 
libaio glib2 bridge-utils iproute gettext
dev86 bin86 iasl markdown git wget

optional packages:  ocaml ocaml-findlib

You will need to enable the 'extra' repository to get bin86. A tool such as yaourt or packer can aid in downloading, compiling and installing dependencies for AUR packages.

Configuring Xen

The following configuration steps are required once the Xen package is installed.

The dom0 host requires

• an entry in the bootloader configuration file
• systemd services to be started at boot time
• a xenfs filesystem mount point
• bridged networking configuration

In addition to these required steps, the current xen.org wiki has a section regarding best practices for running Xen. It includes information on allocating a fixed amount of memory dom0 and how to dedicate (pin) a CPU core for dom0 use.

Bootloader Configuration

Xen requires that you boot a special xen kernel (xen.gz) which in turn boots your system's normal kernel. A new bootloader entry is needed. To boot into the Xen system, we need a new menuentry in grub.cfg. The Xen package provides a grub2 generator file: /etc/grub.d/09_xen. This file can be edited to customize the Xen boot commands, and will add a menuentry to your grub.cfg when the following command is run:

grub-mkconfig -o /boot/grub/grub.cfg

Example non-xen menuentry for LVM with gpt partition table

menuentry 'Arch ' {
  insmod part_gpt
  insmod lvm
  insmod ext2
  set root='lvm/vg0-arch'
  linux /boot/vmlinuz-linux root=/dev/mapper/vg0-arch ro init=/usr/lib/systemd/systemd quiet
  initrd /boot/initramfs-linux.img
}

The menuentry to boot the same arch system after Xen has been installed. Get the UUID for lvm/vg0-arch by using blkid.

menuentry 'Arch Xen 4.2' {
  insmod lvm
  insmod part_gpt
  insmod ext2
  set root='lvm/vg0-arch'
  search --no-floppy --fs-uuid --set=root 346de8aa-6150-4d7b-a8c2-1c43f5929f99
  multiboot /boot/xen.gz placeholder dom0_mem=1024M
  module /boot/vmlinuz-linux placeholder root=/dev/mapper/vg0-arch ro init=/usr/lib/systemd/systemd quiet
  module  /boot/initramfs-linux.img
}

Example for a physical partition

Arch Linux(XEN)
menuentry "Arch Linux(XEN)" {
    set root=(hd0,X)
  search --no-floppy --fs-uuid --set=root 346de8aa-6150-4d7b-a8c2-1c43f5929f99
    multiboot /boot/xen.gz dom0_mem=1024M
    module /boot/vmlinuz-linux-xen-dom0 root=/dev/sda ro
    module /boot/initramfs-linux-xen-dom0.img
}

More at Grub2

Systemd Services

Issue the following commands as root so that the services are started at bootup:

# systemctl enable xenstored.service
# systemctl enable xenconsoled.service
# systemctl enable xendomains.service

Xenfs Mountpoint

Include in your /etc/fstab

 none /proc/xen xenfs defaults 0 0

Bridged Networking

Previous versions of Xen provided a bridge connection whereas Xen 4.2 requires that network communications between the guest, the host (and beyond) is set up separately. The use of both DHCP and static addressing is possible, and the choice should be determined by your network topology. With basic bridged networking, a virtual switch is created in dom0 that every domu is attached to. More complex setups are possible, see the Networking article on the Xen wiki for details.

Netcfg greatly simplifies network configuration and is now included as standard in the base package. Example configuration files are provided in etc/network.d/examples and Xen 4.2 provides scripts for various networking configurations in /etc/xen/scripts.

By default, Xen expects a bridge to exist named xenbr0. To set this up with netcfg, do the following:

# cd /etc/network.d
# cp examples/bridge xenbridge-dhcp

make the following changes to xen-bridge:

INTERFACE="xenbr0"
BRIDGE_INTERFACE="eth0" # Use the name of the external interface found with the 'ip link' command
DESCRIPTION="Xen bridge connection"

assuming your existing eth0 connection is called eth0-dhcp, edit /etc/conf.d/netcfg

NETWORKS=(eth0-dhcp xenbridge-dhcp)

restart the network:

systemctl restart netcfg.service

when the prompt returns, check all is well

ip addr show
brctl show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN 
   link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
   inet 127.0.0.1/8 scope host lo
   inet6 ::1/128 scope host 
      valid_lft forever preferred_lft forever
3: xenbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP 
   link/ether 00:1a:92:06:c0:c0 brd ff:ff:ff:ff:ff:ff
   inet 192.168.1.3/24 brd 192.168.1.255 scope global xenbr0
   inet6 fe80::21a:92ff:fe06:c0c0/64 scope link 
      valid_lft forever preferred_lft forever

bridge name	bridge id		STP enabled	interfaces
xenbr0		8000.001a9206c0c0	no		eth0

Final Steps

Reboot your dom0 host and ensure that the Xen kernel boots correctly and that all settings survive a reboot. A properly set up dom0 should report show the following when you run xl list (as root):

# xl list
Name                                        ID   Mem VCPUs	State	Time(s)
Domain-0                                     0   511     2     r-----   41652.9

Of course, the Mem, VCPUs and Time columns will be different depending on machine configuration and uptime. The important thing is that dom0 is listed.

Using Xen

Once the dom0 is fully operational, domUs may be created / imported. Each OS has a slightly different method of installation, see the Guest Install page of the Xen wiki for links to instructions.

Creating a Paravirtualized (PV) Arch domU

This is how to install Arch as a user domain (or VM) on an already-running Xen host. To install Arch as the Xen host (dom0), see the previous section.

To begin, download the latest install ISO from the nearest mirror: Dowload page. Place the ISO file on the dom0 host. (it is recommended that its checksum be verified, too)

Create the hard disks for the new domU. This can be done with LVM, raw hard disk partitions or image files. To create a 10GiB blank hard disk file, the following command can be used:

truncate -s 10G sda.img

This creates a sparse file, which grows (to a maximum of 10GiB) only when data is added to the image. If file IO speed is of greater importance than domain portability, using a Logical Volume or raw partition may be a better choice.

Next, loop-mount the installation ISO. To do this, ensure the directory /mnt exists and is empty, then run the following command (being sure to fill in the correct ISO path):

# mount -o loop /path/to/iso /mnt

Create the bootstrap domU configuration file:

/etc/xen/archdomu.cfg

kernel = "/mnt/arch/boot/x86_64/vmlinuz"
ramdisk = "/mnt/arch/boot/x86_64/archiso.img"
extra = "archisobasedir=arch archisolabel=ARCH_201301"
memory = 256
name = "archdomu"
disk = [ "phy:/path/to/partition,sda1,w", "file:/path/to/ISO,sdb,r" ]
vif = [ 'mac=00:16:3e:__random_three_mac_bytes__,bridge=xenbr0' ]

This file needs to tweaked for your specific use. Most importantly, the archisolabel=ARCH_201301 line must be edited to use the release year/month of the ISO being used. If you want to install 32-bit Arch, change the kernel and ramdisk paths from /x86_64/ to /i686/. The "phy:/path/to/partition,sda1,w" line must be edited to point to the partition created for the domU. If an image file is being used, the phy: needs to be changed to file:. Finally, a MAC address must be assigned. The 00:16:3e MAC block is reserved for Xen domains, do the last three digits may be randomly filled in (hex values 0-9 and a-f only). See the xl.cfg man page for more information on what the .cfg file lines do. The AUR package xen-docs will need to be installed to access the man pages.

Create the new domU:

# xl create -c /etc/xen/archdomu.cfg

The -c option will enter the new domain's console when successfully created. At this point, Arch should be installed as usual. The Installation Guide should be followed. There will be a few deviations, however. The block devices listed in the disks line of the cfg file will show up as /dev/xvd*. Use these devices when partitioning the domU. After installation and before the domU is rebooted, the following modules must be added to /etc/mkinitcpio.conf:

MODULES="xen-blkfront xen-fbfront xen-netfront xen-kbdfront"

Without these modules, the domU will not boot correctly. After saving the edit, rebuild the initramfs with the following command:

mkinitcpio -p linux

For booting, it is not necessary to install Grub. Xen has a Python-based grub emulator, so all that is needed to boot is a grub.cfg file: (It may be necessary to create the /boot/grub directory)

/boot/grub/grub.cfg

menuentry 'Arch GNU/Linux, with Linux core repo kernel' --class arch --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-core repo kernel-true-__UUID__' {
        insmod gzio
        insmod part_msdos
        insmod ext2
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  __UUID__
        else
          search --no-floppy --fs-uuid --set=root __UUID__
        fi
        echo    'Loading Linux core repo kernel ...'
        linux   /boot/vmlinuz-linux root=UUID=__UUID__ ro
        echo    'Loading initial ramdisk ...'
        initrd  /boot/initramfs-linux.img
}

This file must be edited to match the UUID of the root partition. From within the domU, run the following command:

# blkid

Replace all instances of __UUID__ with the real UUID of the root partition (the one that mounts as "/").

Shutdown the domU with the poweroff command. The console will be returned to the hypervisor when the domain is fully shut down, and the domain will no longer appear in the xl domains list. Now the ISO file may be unmounted:

# umount /mnt

The domU cfg file should now be edited. Delete the "kernel = ", "ramdisk = ", and "extra = " lines and replace them with the following line:

bootloader = "pygrub"

Also remove the ISO disk from the "disk = " line.

The Arch domU is now set up. It may be started with the same line as before:

# xl create -c /etc/xen/archdomu.cfg

If the domU should be started on boot, create a symlink to the cfg file in /etc/xen/auto and ensure the xendomains service is set up correctly.

Useful xl command examples

# xl top
# xl list
# xl console domUname
# xl shutdown domUname
# xl destroy domUname

Common Errors

• 'xl list' complains about libxl

- Either you have not booted into the Xen system, or xen modules listed in xencommons script are not installed

• xl create fails

- check the guest's kernel is located correctly, check the pv-xxx.cfg file for spelling mistakes (like using initrd instead of ramdisk)

• Arch linux guest hangs with a ctrl-d message

- press ctrl-d until you get back to a prompt, rebuild its initramfs described

• Error message "failed to execute '/usr/lib/udev/socket:/org/xen/xend/udev_event' 'socket:/org/xen/xend/udev_event': No such file or directory"

- caused by /etc/udev/rules.d/xend.rules; xend is (a) deprecated and (b) not used, so it is safe to remove xend.rules

Resources

• The homepage at xen.org
• The wiki at xen.org