Xen Project Hypervisor: Difference between revisions
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Revision as of 22:09, 30 October 2024
The Xen Project Hypervisor 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 Domain 0, and can paravirtualise or fully virtualise Unprivileged Domains (domUs
).
Xen is well-known for its impeccable security record, and is the go-to solution for hyper-scale cloud infrastructures.
Installation
Since NixOS 24.11, installing the Xen Hypervisor is as simple as adding the following to your NixOS configuration:
configuration.nix
{
virtualisation.xen.enable = true;
}
After a successful reboot, you should now be using a Xen EFI kernel, and Xen's usual commands, such as xl
, 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 host machine, 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 xl
command. See the manual page xl(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.
Configuration
There are many options available for configuring the Domain 0. Here is a recommended non-default configuration:
configuration.nix
{
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.
};
};
}
Running VMs
Currently, unprivileged domains can only be created/destroyed imperatively. See the usual 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:
example-hvm.cfg
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'
See xl.cfg(5)
for more configuration options.
You can then start the domain using the following command:
# xl create /path/to/example-hvm.cfg -Fc
See also
- The
virtualisation.xen.*
option set.