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This document explains how to set up 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 and allows several operating systems to run simultaneously on top of it.  Once the Xen hypervisor is loaded, it starts the "Dom0" (for "domain 0"), or privileged domain, which in our case runs a Linux kernel (other possible Dom0 operating systems are NetBSD and OpenSolaris). The hardware must, of course, be supported by this kernel to run Xen. Once the Dom0 has started, one or more "DomU" domains can be started and controlled from Dom0.
Xen.org provide a [http://wiki.xen.org/wiki/Xen_Overview 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 as they do not have to run in emulated hardware.
===Hardware Virtual (HVM)===
For hardware virtualization in our domUs, 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.
=== Paravirtual on Hardware (PV on HM) ===
There is a third mode which runs  Xen on top of a HardwareVirtual guest.
=== Recommended Practices ===
The [http://wiki.xen.org/wiki/Main_Page 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 its own use.
== Obtaining Xen ==
Xen is available from [https://aur.archlinux.org/packages.php?ID=14640 AUR], it provides the means to create a Xen host. The Xen host maintains the tools and configuration files for creating and controlling Xen guests.
=== Before installation ===
Like all AUR packages, the binary is built on your machine. For Xen, an internet connection is needed during its compilation because further source files are downloaded during the process. Xen.org recommend 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 base-devel 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.
=== After installation ===
The following steps are required after installation , most consist of adding a line or two to a configuration file, or commenting out others. All of these requirements are fully covered in this document.
'''The dom0 host requires'''
* systemd services to be started at boot time and additional configuration for
* a xenfs filesystem mount point,
* an entry in the bootloader configuration file
* additional networking configuration
* a configuration file for each guest
* a means of accessing each guest's kernel and initrd image.
'''Each domU guest needs'''
* to be installed (refer to the distro's installation guide)
'''Each paravirtualized (pv) guests needs'''
* a mount point corresponding to the virtual disk (specified in its ''etc/fstab'' or equivalent)
* tty1 replaced by a virtual console (specified in its ''etc/inittab'' or equivalant, or systemd service file).
To speed the introduction of 4.2, the maintainer during Xen 4.1 stepped aside; there are significant changes between 4.1 and 4.2, coupled with the transition of Arch from rc.d to systemd. It may take a short time for the new package to settle out so, for the moment, a section on building Xen from source is provided near the end.
== 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. See [[#Bootloader Configuration]].
The menuentry for a Xen system starts a Xen kernel before starting the main host's kernel.
=== grub2 ===
To boot into the Xen system, we need a new menuentry in 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 [https://wiki.archlinux.org/index.php/Grub2 Grub2]
== Network Configuration ==
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. Using dhcp throughout simplifies things while we get everything working, the guest. When  fully working, the guest will normally benefit from a static network address.
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''.
=== Network Bridge ===
By default, Xen expects a bridge connection to be named xenbr0.
# cd /etc/network.d
# cp examples/bridge xenbridge-dhcp
make the following changes to xen-bridge:
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 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 brd 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 preparations to start Xen at boot ==
Include in your ''/etc/fstab''
  none /proc/xen xenfs defaults 0 0
Issue the following so that the services are started at bootup:
# systemctl enable xenstored.service
# systemctl enable xenconsoled.service
# systemctl enable xendomains.service
'''Reboot into your new Arch Dom0.'''
== Creating guest domains (domU) ==
=== Creating Paravirtualized (PV) Guests===
The general procedure is:
perform a normal or minimal installation of the distro that will become a guest; copy its kernel/initrd to a directory on the host; modify its /etc/fstab to use the virtual disk; modify its the way it sets up a terminal (getty); create a config file for xl.
== Running guest Domains ==
Start the guest domU and a console
# xl create /etc/xen/pv-squeeze.cfg
Check all is well:
# xl list
=== Useful xl command examples ===
# xl top
# xl list
# xl shutdown pv-squeeze
# xl destroy pv-squeeze
== Worked example for Debian Squeeze as a guest ==
Install Debian 6.0 (do not bother with graphical interface, install as little as possible). Having installed it, boot into in your new Arch Xen system and mount it.
The example has the guest Debian Squeeze installed onto /dev/vg0/pv_squeeze
# mkdir /tmp/squeeze
# mkdir -p /var/lib/xen/images/squeeze
# mount -text4 /dev/vg0/pv_squeeze /tmp/squeeze/
Copy the kernel and initrd to a location available Xen. n.b. Squeeze has softlinks (vmlinuz and initrd.img) in its root directory to the current kernel, so check you have copied a real kernel, and not just a link!
# cp /tmp/squeeze/vmlinuz /tmp/squeeze/initrd.img /var/lib/xen/images/squeeze
edit /tmp/squeeze/etc/fstab
change its root entry to begin with /dev/xvda1
# /dev/xvda1 / ext4 noatime,nodiratime,errors=remount-ro 0 1
Debian Squeeze uses ''/etc/inittab'' to configure its terminals. Other distros use other mechanisms. We need to replace the creation of terminals ''tty1'', ''tty2'' etc. with a single ''hvc0''.
Comment out any ''getty tty'' lines like these:
1:2345:respawn:/sbin/getty 38400 tty1
2:23:respawn:/sbin/getty 38400 tty2
Replace with the single line
hvc:2345:respawn:/sbin/getty 38400 hvc0
Create a guest configuration file by copying one of the given example files and editing as follows:
# cp /etc/xen/xlexample.pvlinux /etc/xen/pv-squeeze.cfg
edit /etc/xen/pv-squeeze.cfg with the following:
name = "squeeze.pvlinux"
extra = "root=/dev/xvda1"
memory = 256
vcpus = 2
disk = [ '/dev/vg0/pv_squeeze,raw,xvda1,rw' ]
Start the guest domU and a console
# xl create /etc/xen/pv-squeeze.cfg
Check all is well:
# xl list
Name            ID  Mem VCPUs    State Time(s)
Domain-0        0  1024    2    r-----      26.2
squeeze.pvlinux  1    123    2    -b----      1.5
Start a console:
# xl console squeeze.pvlinux
( example output)
    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Linux version 2.6.32-5-xen-amd64 (Debian 2.6.32-46) ...UTC 2012
[    0.000000] Command line: root=/dev/xvda1
[    0.000000] KERNEL supported cpus:
[    0.000000]  Intel GenuineIntel
[    0.000000]  AMD AuthenticAMD
[    0.000000]  Centaur CentaurHauls
== Installation notes for domU Paravirtualized guests ==
=== Arch ===
The default Arch initramfs images lack essential xen modules.
In the guest install, we need to add the following to mkinitcpio.conf
MODULES = "xen-blkfront xen-fbfront xen-netfront xen-kbdfront"
and then rebuild its initramfs-linux.img with ''mkinitcpio -p linux''.
=== Debian ===
Installation for Wheezy (testing) is identical to that for Squeeze (stable), see [[#Worked example for Debian Squeeze as a guest]].
== 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
* The domu guest hangs at 'crond'
- The guest's terminal needs to be set to ''hvc0'' instead of ''tty1'' See the Debian Squeeze example above.
* 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
== Building and Installing Xen Hypervisor and Dom0 Host from Source ==
Xen recommends that a Xen host (dom0) is 64-bit, guests may be either 32-bit or 64-bit. To build such a system requires a mixed 64/32-bit installation and packages from the the Community repository; the host uses a network bridge and a modified entry in the bootloader configuration file (for example, grub.cfg). These notes assume an installation using systemd is in use, as is the default for a new installation of Arch. For these reasons, you may prefer to make a fresh installation of Arch on which to build and install Xen.
===Building Xen===
Building and installing Xen significantly modifies your system. Xen is an established program, but Xen 4.2 is extremely new. Consider Xen 4.2 on an Arch system to be untested. Consider yourself to be an alpha tester, perhaps make a throw-away Arch system for the Xen installation.
'''It is best practise to backup and highly recommended to make a fresh installation of Arch on which to build and install Xen.'''
* The build process installs additional source from git, so a working internet connection is required.
* systemd service files will be available soon. Until then we use (the currently still supported, but legacy) rc.d and rc.conf.
Edit /etc/pacman.conf to uncomment entries under repositries for multilib and community (three lines each).
Prepare for and perform a full system upgrade (pacman -Syu).
Install packages listed under 'Obtaining Xen'.
Download Xen Hypervisor 4.2 tarball from http://xen.org/products/downloads.html.
Unpack the tarball to a suitable location (tar xjf <path/to/tarball> location). Unfortunately, the build process also creates binaries and scripts for the old, deprecated ''xend/xm''.
The Xen documentation recommends building Xen as root.
# cd xen-4.2.0
# PYTHON=/usr/bin/python2
# export PYTHON
# ./configure
# make world
# make dist
The dist directory can be used to install Xen to any machine, but it
* sets the 'sticky' bit on all file permissions
* installs startup scripts to ''etc/init.d'' (equivalent of ''etc/rc.d'')
* includes some udev rules for 'xend' which creates LOTS of error messages when booting up (xend is not used, having been replaced by xendomains)
The only script we need from ''etc/init.d'' is ''xendomains'' since the systemd service files given below replace ''etc/init.d/xencommons''. The service files are based on those in Fedora 17 (which uses systemd and provides Xen 4.1). However, it places ''xendomains'' in ''/usr/libexec'' which is not present in Arch. The ''xendomain.service'' below uses ''/etc/xen/scripts'' as the location for ''xendomains''.
Fix these problems with
# cd dist
# chmod -R -s install/
# cp install/etc/init.d/xendomains install/etc/xen/scripts
# rm install/etc/init.d/*
# rmdir install/etc/init.d
# rm install/etc/udev/rules.d/xend.rules
If installing to another Arch system, make a tarball and copy it over:
# cd ..
# tar cjf ~/xen-dist-4.2.bz2 dist/
copy the tarball to the other installation, boot into it
use 'tar xjf xen-dist-4.2.bz2 .' to unpack
then install packages listed under 'Packages required for host'
Now change to the 'dist' directory and install
# cd dist
Whether installing now, or to another installation, from the ''dist'' directory issue:
# ./install.sh
=== Enabling Xen under Systemd ===
Add the following files
n.b The following were included in xencommons, but were not inserted by systemd
''evtchn'' ''gntdev'' ''netbk'' ''blkbk'' ''xen-scsibk'' ''usbbk'' ''pciback'' ''blktap2'' ''blktap''.
''xen-acpi-processor'' may not work on some machines. Remove this when getting error.
Description=Xenstored - daemon managing xenstore file system
Before=libvirtd.service libvirt-guests.service
ExecStart=/usr/sbin/xenstored --pid-file /var/run/xenstored.pid $XENSTORED_ARGS
Description=Xenconsoled - handles logging from guest consoles and hypervisor
Description=Xendomains - start and stop guests on boot and shutdown
Requires=proc-xen.mount xenstored.service
After=proc-xen.mount xenstored.service xenconsoled.service
ExecStartPre=/usr/bin/grep -q control_d /proc/xen/capabilities
ExecStart=/etc/xen/scripts/xendomains start
ExecStop=/etc/xen/scripts/xendomains stop
  Description=Mount /proc/xen files
Issue the following so that the services are started at bootup:
# systemctl enable xenstored.service
# systemctl enable xenconsoled.service
# systemctl enable xendomains.service
Add a new Xen menuentry in grub.cfg as described earlier and then '''reboot into the new Xen system''' and check all is well:
# xl list
Name                                        ID  Mem VCPUs State Time(s)
Domain-0                                    0  1024    2    r-----      6.1
n.b. ''xencommons'' sets the name "Domain-0" in the xenstored database. The current systemd service files do not do this, so at the moment ''xl list'' displays "(null)" as the name for Dom0.
* [http://www.xen.org/ The homepage at xen.org]
* [http://wiki.xen.org/wiki/Main_Page The wiki at xen.org ]

Revision as of 14:01, 16 April 2013

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