Xen - The Low-Level Hypervisor

Discussion in 'Linux Kernel' started by DevynCJohnson, Aug 3, 2014.

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Have you ever used Xen? If so, did you like it?

  1. No, I never used Xen

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  2. Yes, I like Xen

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  3. Yes, but I do not like Xen

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  1. DevynCJohnson

    DevynCJohnson Well-Known Member Staff Member Staff Writer

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    Linux Kernel - http://www.linux.org/threads/linux-kernel-reading-guide.5384/

    For those of you that understand virtualization, you may want to have an understanding of the most popular hypervisor for Linux - Xen. Xen is an open-source hypervisor that is well supported by Linux. Some other operating systems support Xen as well. Knowing the basics of Xen can help you understand hypervisors better and know why Xen is so popular.

    Xen is a popular hypervisor that supports OS virtualization and paravirtualization (PV). Xen is a "Type 1" hypervisor meaning that it runs on the hardware. When the computer boots-up, the bootloader (typically GRUB) loads Xen (Xen is a microkernel). Then, Xen sets up specialized virtual machines called "Domains". Domains are low-level virtual machines that do not have a host, and only one domain can access the hardware directly. Xen first sets up the primary domain. This domain is known by many names such as "Domain Zero", "Dom0", "Domain0", "Host Domain", or "The Control Domain". Xen cannot function without the control domain. The control domain provides the drivers that are used by itself and other domains. The control domain also contains the controls and settings for Xen as well as Xen itself. The other domains are called "Guest Domains", "Unprivileged Domains", or "DomU". The guest domains use the drivers provided by Domain0. The guest domains are not permitted to access the hardware directly. Instead, they must allow Dom0 to manage the hardware. (http://www.xenproject.org/)(http://wiki.xen.org)

    NOTE: With a virtualization structure like this, it is hard to specifically say which system is the host. Some may argue that Xen is the host system since it is a layer between the hardware and operating systems. As for the other view, Domain0 could be considered the host since Xen depends on it and unprivileged domains need Domain0 for hardware access. Either way, the unprivileged domains act like guests, so they are often called guest domains. Some manuals may refer to the hardware itself as the host.

    Most Linux distros contain PV-enabled kernels; PV is enabled in the vanilla kernel. PV-enabled kernels contain the paravirt_ops (pv-ops) framework which is a kernel infrastructure that provides hypercalls and other components needed for paravirtualization. Thus, paravirtualization does not require virtualization support from the CPU. Just as the userspace sends system calls to the kernel, the Linux kernel sends hypercalls to Xen. Hypercalls usually have a naming scheme like this "__HYPERVISOR_*", where the asterisk represents the rest of the hypercall's name. (http://xenbits.xen.org/docs/unstable/hypercall/index.html)

    NOTE: Other hypervisors recognize and use the hypercalls from the paravirt_ops framework.


    Xen can send the virtual operating systems "Virtual Interrupts". These virtual interrupts go to the kernel of the virtual OS. The names of the interrupts begin with (VIRQ_). "Event Channels" provide the framework for event notifications. These notifications are Xen's equivalent to hardware interrupts.

    Xen Cloud Platform (XCP) is an ISO file that comes with Xen and a Linux distro set as Dom0. In other words, this is a complete Xen system that you install from the ISO. Otherwise, install a Linux distro and use the package manager to install Xen. If you are interested in installing Xen, then I recommend you go to Xen's official website and read the installation instructions. (http://wiki.xenproject.org/wiki/Xen_Project_Beginners_Guide)

    Often times, Xen systems use a Logical Volume Manager (LVM) for local storage, but this is not required. Xen can also use network storage. Xen also uses “Xen Virtual Block Devices” (VBD or XVD). These are files where the file itself is a virtual hard-drive. Such files use the “.dsk” file extension. The first virtual Xen hard-drive is designated as /dev/xvda and /dev/xvdb is the second (just change the last letter). The virtual hard-drives are treated as IDE drives, but number of maximum partitions is fifteen. Xen Virtual Block Devices are commonly used with paravirtualized domains.

    NOTE: Domain0 must be installed on the physical hardware, but all of the guest domains may be installed in XVDs.

    You may be wondering, “why install domains in an XVD file?”. Well, doing so allows the virtual OS to be copied as a file. Admins can copy the “.dsk” file and easily and quickly install preconfigured systems or make backups. Having a complete OS in a single file makes backups and installation very easy. Since Xen is a hypervisor, nearly any system running Xen would be able to use the OS in the “.dsk” file. This also makes hardware requirements flexible because as long as Xen is supported and Domain0 has the needed drivers, a particular virtual OS will be able to run on any machine. If an OS becomes corrupted, the file can be copied over the top of the corrupted Domain's image.

    LINK: This site offers some pre-made Xen images as well as images for other hypervisors. http://stacklet.com/downloads/images/public

    XenServer is an open-source Xen system offered by Citrix. Users can purchase support, but the software itself is free. XenServer has an interesting piece of software called XenMotion. XenMotion allows a currently running guest domain to be moved from one physical machine to another host machine that both share the same storage. This allows admins to repair servers without having the system down. (http://xenserver.org/)

    Xen supports various operating systems including Windows. However, Windows can only be in a guest domain, never in the control domain. NetBSD and OpenSolaris are two examples of operating systems that can function as Domain0.

    "xencommons" is the init script that each operating system domain must have properly installed to run the required Xen services.

    Various interfaces exist for managing a Xen system. Users can use Xen's command-line tools or install a separate interface. Many management tools exist including graphical interfaces. "libvirt" is an example of a hypervisor management API that many management tools use. "libvirt" is a library written in C, but various programming bindings allow programmers to implement libvirt in management tools that are programmed in other computer languages. (http://wiki.xen.org/wiki/Xen_Management_Tools)(http://wiki.xen.org/wiki/XCP_Management_Tools)

    Xen uses the Hierarchical Protection Domains (CPU Protection Rings) security feature offered by most (or all) processors. A CPU can execute code under different security modes. Most CPUs have four rings (security modes) designated "Ring 0" through "Ring 3". Ring 0 is the most privileged and allows complete direct-access to the hardware. In contrast, Ring 3 has very few privileges and must request permission from the user to perform special tasks. Xen runs in Ring 0 while Dom0 and the guest domains all run in Ring 1. User applications run in Ring 3, which is the default for most operating systems. Xen does not assign any process to Ring 2.


    Xen is a powerful hypervisor with many features and uses. Having a general understanding of Xen can help admins decide which hypervisor to use. Understanding Xen also helps users gain a better understanding of virtualization and Linux.

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