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Introduction

Xen is Blue Light's standard virtualisation solution, decided in BLUE-1192

Introduction to Xen: http://wiki.xenproject.org/wiki/Xen_Beginners_Guide#What_is_this_Xen_Project_software_all_about.3F

Installation (host/dom0)

TODO: better/different to install xen-linux-system instead of xen-tools?

aptitude install xen-linux-system

Adjust grub so the default boot item is Xen (ref: https://xen-orchestra.com/cant-find-hypervisor-information-in-sysfs/)

dpkg-divert --divert /etc/grub.d/08_linux_xen --rename /etc/grub.d/20_linux_xen
update-grub
shutdown -r now

Test: does the  xen list  command show that the current system is Domain-0?

Toolstacks (a.k.a toolkits and toolboxes)

There are several toolstacks available for the personal or scripted administration of Xen resources.

Primary reference: http://wiki.xen.org/wiki/Choice_of_Toolstacks

Apparently, once a toolstack has been chosen, it is not easy to change.  From Debian 7 Wheezy's /etc/default/xen (where the chosen toolstack is configured):

# Attention: You need to reboot after changing this!

libvirt and virsh

libvirt is a library for managing KVM, OpenVZ, VMware, VirtualBox, Xen and others.  For Xen, it is the least feature-complete of all the toolstacks.

virsh is a user/script shell to interface with libvirt.  References: http://libvirt.org/virshcmdref.html

xapi and xe

The most feature-complete of all the toolstacks.

xend and xm

Deprecated since Xen 4.1 but the default in Xen 4.1 as packaged for Debian.  Will be removed from Xen 4.2.

xend comprises:

  • xm command

  • xend-config.sxp config file format

  • xmdomain.cfg config file format

xend was abandoned because of @@@

xl

Strategic since Xen 4.1.

Designed to be command line compatible with xend.  TODO: does it have an xm command?

On Debian 7 Wheezy:

root@SON-OF-blav.bluelightav:~# type xl
xl is /usr/sbin/xl
root@SON-OF-blav.bluelightav:~# ll /usr/sbin/xl
lrwxrwxrwx 1 root root 43 May 5 2013 /usr/sbin/xl -> ../lib/xen-common/bin/xen-toolstack-wrapper

Toolstack library

Libxenlight (libxl)

The strategic Xen toolstack library, designed to hide xenstore, libxenctrl, and libxenguest from higher levels.

Toolstack configuration

Debian 7 Wheezy

The Xen toolstack on Debian 7 Wheezy is configurable.

On blav2, we had explicitly configured using xl:

root@SON-OF-blav.bluelightav:~# diff /etc/default/xen{.org,}
7c7 < TOOLSTACK=
--- > TOOLSTACK=xl

When root runs command xl:

  1. The shell runs /usr/sbin/xl which is a symlink to /usr/lib/xen-common/bin/xen-toolstack-wrapper
  2. /usr/lib/xen-common/bin/xen-toolstack-wrapper:
    1. Sources /usr/lib/xen-common/bin/xen-toolstack to set TOOLSTACK to /usr/lib/xen-4.1/bin/xl
    2. Sources /usr/lib/xen-common/bin/xen-dir to set dir to /usr/lib/xen-4.1
    3. Sources /usr/lib/xen-common/bin/xen-version to set VERSION to 4.1
      1. /usr/lib/xen-common/bin/xen-version gets Xen version info from /sys/hypervisor/version/*
    4. Sets PATH to /usr/lib/xen-4.1/bin
    5. Sources /etc/default/xen to set TOOLSTACK to xl (would fall back to hardcoded default otherwise)
    6. Sets TOOLSTACK to /usr/lib/xen-4.1/bin/xl
    7. execs /usr/lib/xen-4.1/bin/xl

When root runs command xm (does not work):

  1. The shell runs /usr/sbin/xm which is a symlink to /usr/lib/xen-common/bin/xen-toolstack-wrapper
  2. /usr/lib/xen-common/bin/xen-toolstack-wrapper:
    1. Sources /usr/lib/xen-common/bin/xen-toolstack to set TOOLSTACK to /usr/lib/xen-4.1/bin/xl
    2. Sources /usr/lib/xen-common/bin/xen-dir to set dir to /usr/lib/xen-4.1
    3. Sources /usr/lib/xen-common/bin/xen-version to set VERSION to 4.1
      1. /usr/lib/xen-common/bin/xen-version gets Xen version info from /sys/hypervisor/version/*
    4. Sets PATH to /usr/lib/xen-4.1/bin
    5. Sources /etc/default/xen to set TOOLSTACK to xl (would fall back to hardcoded default otherwise)
    6. Sets TOOLSTACK to /usr/lib/xen-4.1/bin/xl
    7. Generates message "ERROR: A different toolstack (xl) have been selected!"

Configuration

DomU*

Networking

Choice:

  • Bridged  Each DomU* OS is connected to the LAN
  • Routed  Dom0 is a router between the LAN and the DomU* network interfaces
  • NATted  Dom0 is a NATting router between the LAN and the DomU* network interfaces

 

Installation (domu*)

 

Troubleshooting

TODO: integrate the next para.

A quick note, there are actually 3 modes, not two when it comes to the drivers in use:

  • HVM: unmodified kernel and drivers using software emulated devices
  • PV-HVM: unmodified kernel with paravirtualized (Xen specific) disk and network drivers
  • PV: modified kernel and drivers

For a Xen guest/DomU you can do a very basic uname and lsmod with a grep to list the modules in use:

uname -a lsmod | grep xen

If uname -a lists a kernel with the string "xen" in it, then you have a modified kernel and it's likely a PV guest, and you will see output from the lsmod command to confirm it. If you have output from the grep on lsmod but no sign of a modified kernel then you are PV-HVM. Without any sign of either, it's a straight HVM.

Note: Generally you can do more with VMs that have the PV tools installed, so that can be quite an obvious pointer, however you can fake the presence of the PV tools to allow suspend/resume etc. so you cannot rely on that in general.

References

Files and directories

File or directoryUsageNotes
/etc/default/xen  
/etc/default/xend  
/etc/default/xendomains  
​/etc/xen/​Configuration​
/etc/xen/scripts/? 
/etc/xen/scripts/vif-nat  

Glossary

  • Domain  a Xen VM.
  • dom0  The Xen domain/VM used a) to control the hypervisor b) as the primary interface to the hardware.
  • domu*  Unprivileged Xen domains/VMs
  • Enlightened guest  An OS designed/modified for use in a domain/VM.
  • Full virtualisation  A domain/VM providing full emulation of physical hardware.  The OS can be the same as runs on real hardware.
  • Guest domain  Same as domu*.
  • Hardware Virtual Machine (HVM)  Same as full virtualisation.
  • Hypervisor a software system that allows the execution of multiple virtual guest operating systems simultaneously on a single physical machine. Xen's hypervisor is Type 1 or “bare-metal”, meaning it runs on the physical machine as opposed to within an operating system.
  • HVM  Hardware Virtual Machine.
  • Para-virtualization  uses modified guest operating systems a.k.a. enlightened guests. They don’t require virtual hardware devices, instead they make special calls to the hypervisor that allow them to access CPUs, storage and network resources.  Better performance that full virtualisation.
  • PV  para-virtualisation.
  • Virtual Machine Monitor (VMM)  Same as hypervisor.
  • Xen Store  A database of information shared by the hypervisor, the kernels, the drivers and the xen daemon.  The memory for Xen Store is provided by Dom0.  In Dom0 (and in enlightened guests?) it can be accessed via /proc/xen/xenbus/

Technical

TODO; move Xen notes from the Online.net server page to here.

Architecture

Very little information found on the 'net about Xen architecture.  This diagram is from http://libvirt.org/architecture.html

TODO: mine http://wiki.xen.org/wiki/XenIntro for more information.

Primary reference (no Xen 4.x equivalent found): http://wiki.xen.org/wiki/XenIntro

Debian packages

libxen-4.1

Includes /usr/lib/libxenctrl-4.1.so and /usr/lib/libxenguest-4.1.so

libxenstore3.0

Includes /usr/lib/libxenstore.so.3.0.0 and /usr/lib/libxenstore.so.3.0

xen-hypervisor-4.1-amd64

Includes /boot/xen-4.1-amd64.gz

xen-linux-system-3.2.0-4-amd64

Documentation only.

xen-linux-system-amd64

Documentation only.

xen-system-amd64

Documentation only.

xen-tools

Tools that work with the xm and xl toolstacks to administer Xen config files and images.

Provides (in /usr/bin):

  • xt-customize-image
  • xt-create-xen-config
  • xen-list-images
  • xen-delete-image
  • xt-install-image
  • xen-create-nfs
  • xen-create-image
  • xt-guess-suite-and-mirror
  • xen-update-image

Plus, presumably in suppport:

  • /etc/xen-tools/*
  • /usr/lib/xen-tools/<OS name>.d/*

Logs in the /var/log/xen-tools/ directory.

References

xen-utils-4.1

Provides (in /usr/lib/xen-4.1/bin/):

  • xentrace_setsize
  • xenpm
  • xen-python-path
  • qemu-dm
  • xsview
  • xen-hvmcrash
  • xentrace_setmask
  • xenstored
  • xentrace_format
  • xenbaked
  • readnotes
  • xenconsoled
  • xc_restore
  • gtraceview
  • xen-bugtool
  • xen-hptool
  • xentop
  • xend
  • xc_save
  • gtracestat
  • xenctx
  • xenwatchdogd
  • xenpaging
  • pygrub
  • xencons
  • xen-detect
  • lsevtchn
  • tap-ctl
  • xl
  • xentrace
  • xm
  • xen-hvmctx
  • xenconsole
  • xenstore-control
  • xenmon
  • xenlockprof
  • xen-tmem-list-parse
  • xenperf

Plus, presumably in suppport:

  • /usr/lib/xen-4.1/lib/
  • /usr/lib/xen-4.1/boot/hvmloader

xen-utils-common

Provides:

  • /etc/default/xend
  • /etc/default/xendomains
  • /etc/init.d/xen
  • /etc/init.d/xendomains
  • /etc/xen/scripts/*
  • /etc/xen/xend-config.sxp
  • /etc/xen/xend-pci-permissive.sxp
  • /etc/xen/xend-pci-quirks.sxp
  • /etc/xen/xl.conf
  • /lib/udev/rules.d/xen-backend.rules
  • /lib/udev/rules.d/xend.rules
  • /usr/lib/xen-common/bin/*
  • /usr/sbin/cpuperf-perfcntr
  • /usr/sbin/cpuperf-xen
  • /usr/sbin/xe
  • /usr/sbin/xenperf
  • /usr/sbin/xenpm
  • /usr/sbin/xentop
  • /usr/sbin/xentrace
  • /usr/sbin/xentrace_format
  • /usr/sbin/xentrace_setmask
  • /usr/sbin/xentrace_setsize
  • /usr/sbin/xl
  • /usr/sbin/xm
  • /usr/share/xen-utils-common/default.xen

xenstore-utils

Provides (all in /usr/sbin):

  • xenstore-ls
  • xenstore-write
  • xenstore-exists
  • xenstore-rm
  • xenstore-chmod
  • xenstore-watch
  • xenstore-list
  • xenstore-read

References

Documentation

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