View source for Xen Project Hypervisor

[[File:Xen-project-logo.png|thumb|The Xen Project Logo]]
The <strong>[https://xenproject.org/ Xen Project Hypervisor]</strong> is an open-source type-1 virtual machine manager, which allows multiple virtual machines, known as domains, to run concurrently with the host on the physical machine. This is unlike a typical type-2 hypervisor, like [[QEMU]], where the virtual machines run as applications on top of the host. NixOS runs as the privileged <b>Domain 0</b>, and can paravirtualise or fully virtualise Unprivileged Domains (<code>domUs</code>).

Xen is well-known for its [https://xenbits.xenproject.org/xsa impeccable security record], and is the go-to solution for hyper-scale cloud infrastructures.

== Installation ==
Since [https://nixos.org/manual/nixos/unstable/release-notes#sec-release-24.11-highlights NixOS 24.11], installing the Xen Hypervisor is as simple as adding the following to your [[Overview_of_the_NixOS_Linux_distribution#Declarative_Configuration|NixOS configuration]]:

{{file|configuration.nix|nix|
<nowiki>
{
  virtualisation.xen.enable = true;
}
</nowiki>
}}
{{Evaluate|boot}}

After a successful reboot, you should now be using a Xen EFI kernel, and Xen's usual commands, such as <code>xl</code>, will begin working. Right after a fresh boot, there's usually only a single domain (virtual machine) running: the Domain 0.

=== About the Domain 0 ===
The Domain 0, generically known as the <b>host machine</b>, is the most important virtual machine in a Xen system. It is responsible for orchestrating the Unprivileged Domains, and housing the [[Linux kernel]] version that interacts with the bare-metal hardware. Here, you can use LibXenLight, Xen's main command-line interface, through the aforementioned <code>xl</code> command. See the manual page {{man|xl|sec=1}} for usage information.

An important security feature of Xen is the ability to disaggregate the responsibilities given to the Domain 0. While it will normally be responsible for hosting Xen's shared database, the Xen Store, this responsibility can instead be assigned to a stubdomain: a special type of lightweight Xen virtual machine that runs a Domain 0 function in an isolated and secure manner.

{{Security Warning|If the Domain 0 is compromised by an attacker, then the entire server running Xen should be considered compromised. It is recommended that the NixOS installation running in the host VM is kept as minimal and up-to-date as possible. Avoid packages with large attack services, such as [[:Category:Desktop_environment|desktop environments]], run every user-facing service in an unprivileged domain, and if you're under a lax [[firewall]], consider denying network access to and from Domain 0.|heading=Domain 0 has special security considerations.}}

== Configuration ==

There are many options available for configuring the Domain 0. Here is a recommended non-default configuration:

{{file|configuration.nix|nix|
<nowiki>
{
  virtualisation.xen = {
    enable = true;
    efi.bootBuilderVerbosity = "info"; # Adds a handy report that lets you know which Xen boot entries were created.
    bootParams = [
      "vga=ask" # Useful for non-headless systems with screens bigger than 640x480.
      "dom0=pvh" # Uses the PVH virtualisation mode for the Domain 0, instead of PV.
    ];
    dom0Resources = {
      memory = 1024; # Only allocates 1GiB of memory to the Domain 0, with the rest of the system memory being freely available to other domains.
      maxMemory = 4096; # Allows the Domain 0 to balloon up to 4GiB of memory.
      maxVCPUs = 2; # Allows the Domain 0 to use, at most, two CPU cores.
    };
  };
}
</nowiki>
}}

{{Security Warning|Some option combinations are known to cause security vulnerabilities that may be exploited to cause a denial of service attack. That said, the assertions system present in the <code>xen-dom0.nix</code> module <b>will prevent you from evaluating a known-unsafe configuration.</b>|heading=Some configurations may cause a system to become vulnerable to known security issues.|type=lowPrio}}

== Running VMs ==

Currently, unprivileged domains can only be created/destroyed imperatively. See the usual [https://xenbits.xenproject.org/docs/unstable/ Xen documentation] for more specific usage information. To get you started, here's an example Xen configuration file that can produce a fully virtualised domain:

{{file|example-hvm.cfg|cfg|
<nowiki>
name='example-domain'
memory='2048'
vcpus=2
type='hvm'
disk=[ '/path/to/where/you/want/to/store/the/virtual/disk.qcow2,qcow2,hda,w', 'file:/path/to/a/nixos-installation.iso,hdc:cdrom,r' ]
boot='cd'
</nowiki>
}}

See {{man|xl.cfg|sec=5}} for more configuration options.

You can then start the domain using the following command:

{{Commands|# xl create /path/to/example-hvm.cfg -Fc}}

== See also ==

* The {{nixos:option|virtualisation.xen.*}} option set.

[[Category:Virtualization]]
[[Category:Software]]
[[Category:Server]]
[[Category:Security]]