Virt-manager

Requisites

Even though Virt-manager (using the KVM hypervisor) is able to take advantage of virtualisation capabilities without any UEFI/BIOS configuration, best performances demand that host have Vt-x and Vt-d (Intel) or AMD-V and AMD-Vi (AMD) enabled. These settings can usually be found under the UEFI/BIOS settings.

Installation

NixOS

Before 23.11:

virtualisation.libvirtd.enable = true;
programs.dconf.enable = true; # virt-manager requires dconf to remember settings
environment.systemPackages = with pkgs; [ virt-manager ];

After 23.11:

virtualisation.libvirtd.enable = true;
programs.virt-manager.enable = true;

See relevant merge request

You will get a warning when you open it for the first time:

Could not detect a default hypervisor. Make sure the appropriate QEMU/KVM virtualization packages are installed to manage virtualization on this host.

A virtualization connection can be manually added via File->Add Connection

To resolve:

File (in the menu bar) -> Add connection

HyperVisor = QEMU/KVM
Autoconnect = checkmark

Connect

The same can also be achieved declaratively by setting the corresponding dconf settings with home-manager. If you want to do this instead of the imperative configuration described above, add the following snippet to your home-manager configuration:

dconf.settings = {
  "org/virt-manager/virt-manager/connections" = {
    autoconnect = ["qemu:///system"];
    uris = ["qemu:///system"];
  };
};

You may get the following error:

authentication unavailable: no polkit agent available to authenticate action 'org.libvirt.unix.manage'

To resolve, add the user to the libvirtd group:

{
  users.users.<myuser>.extraGroups = [ "libvirtd" ];
}