VR

Monado

Monado is an open source OpenXR runtime. It offers support for a variety of hardware using its built-in drivers and can be used to run any OpenXR and, with the help of OpenComposite, most OpenVR applications.

Monado can be configured using its NixOS options since 24.05:

/etc/nixos/configuration.nix

services.monado = {
  enable = true;
  defaultRuntime = true; # Register as default OpenXR runtime
};

In order to configure Monado, you might want to add additional environment variables:

/etc/nixos/configuration.nix

systemd.user.services.monado.environment = {
  STEAMVR_LH_ENABLE = "1";
  XRT_COMPOSITOR_COMPUTE = "1";
};

For further information about available environment variables and tweaks, read the Linux VR Adventures wiki and the Monado documentation about environment variables

OpenComposite

OpenComposite is a compatibility layer for running OpenVR applications on an OpenXR runtime like Monado. It is comparable to tools like DXVK or vkd3d, but for translating OpenVR calls to OpenXR.

In order to run OpenVR games on anything other than SteamVR, you need to configure the OpenVR runtime path defined in ~/.config/openvr/openvrpaths.vrpath. A reliable way to do this is to use Home Manager to create this file.

If this file is not set to read-only, SteamVR will add its runtime path back, hence the use for Home Manager.

Warning: Proton will always query the current OpenVR runtime. If you use OpenComposite, and it fails to initialize an OpenXR context, Proton will fail to launch. A workaround is to delete the ~/.config/home-manager/home.nix file and then retry launching the game.

An example configuration for enabling OpenComposite may look like this:

~/.config/home-manager/home.nix

xdg.configFile."openvr/openvrpaths.vrpath".text = ''
  {
    "config" :
    [
      "${config.xdg.dataHome}/Steam/config"
    ],
    "external_drivers" : null,
    "jsonid" : "vrpathreg",
    "log" :
    [
      "${config.xdg.dataHome}/Steam/logs"
    ],
    "runtime" :
    [
      "${pkgs.opencomposite}/lib/opencomposite"
    ],
    "version" : 1
  }
'';

If you are planning to play any OpenVR game on Steam or OpenXR games through Proton, you will have to use OpenComposite in this manner. In most cases you also have to allow access to the socket path of your OpenXR runtime to Steam's runtime, by using the following launch options for XR applications on Steam: env PRESSURE_VESSEL_FILESYSTEMS_RW=$XDG_RUNTIME_DIR/monado_comp_ipc %command%. This example is for Monado, while other XR runtimes might differ.

SteamVR

SteamVR is a proprietary OpenVR runtime with compatibility for OpenXR. It is part of Steam and doesn't need any additional setup on NixOS apart from enabling Steam.

After installing SteamVR through Steam and plugging in a SteamVR-compatible headset, SteamVR should work for the most part.

On initial setup, SteamVR will ask for elevated permissions, to set up a file capability for one of its binaries. This is needed to allow asynchronous reprojection to work. Clients need the CAP_SYS_NICE capability to acquire a high-priority context, which is a requirement for asynchronous reprojection.

Note: Steam is run in a bubblewrap-based FHS environment. This environment runs Steam in a user namespace, which prevents it from using any capabilities or setuid binaries. This means that asynchronous reprojection can not be used on NixOS, without patching the kernel to remove these restrictions completely. See this Nixpkgs issue

Patching AMDGPU to allow high priority queues

By applying this patch, the AMDGPU kernel driver will ignore process privileges and allow any application to create high priority contexts.

Warning: This removes intentional restrictions from the kernel, and it could cause scheduling issues. While it has not been reported that it does cause issues, this should be considered an unsupported configuration.

Applying as a NixOS kernel patch

To workaround the CAP_SYS_NICE requirement, we can apply a kernel patch using the following NixOS configuration snippet:

/etc/nixos/configuration.nix

boot.kernelPatches = [
  {
    name = "amdgpu-ignore-ctx-privileges";
    patch = pkgs.fetchpatch {
      name = "cap_sys_nice_begone.patch";
      url = "https://github.com/Frogging-Family/community-patches/raw/master/linux61-tkg/cap_sys_nice_begone.mypatch";
      hash = "sha256-Y3a0+x2xvHsfLax/uwycdJf3xLxvVfkfDVqjkxNaYEo=";
    };
  }
];

It is also possible to just patch amdgpu and build it as an out-of-tree module, as described in Linux_kernel#Patching_a_single_In-tree_kernel_module

See also

• Linux VR Adventures Wiki