blog/content/posts/nixos-disk-images-m1/index.md

+++ date = "2022-06-21" draft = false path = "/blog/nixos-disk-images-m1" tags = ["nix", "nixos"] title = "Adventures in building disk images of NixOS virtual machines for M1" +++

I work in a thoroughly Nix-based environment, and sometimes I need to test NixOS things, but I have a Mac at work. So, time to get a NixOS VM going on the M1, I guess! This was a partially-planned adventure.

People I know have been saying good things about the (mostly) qemu wrapper UTM (also on GitHub here), so that was the tool of choice.

I kind of didn't realize there were normal installer images that would absolutely just work (since UTM implements UEFI by default), so I tried a more fun option: just build the root filesystem offline and import it.

To do this, you will need a Linux machine with Nix and the capacity to execute aarch64 binaries, either by emulation with binfmt-misc and qemu-user on Linux, or natively because you have an aarch64-linux computer with Nix on it. I have the latter because Oracle Cloud apparently just gives away 24GB memory aarch64 instances for free.

NixOS has infrastructure for building disk images of systems, so it's more or less a case of doing it, with the correct configuration.

The config for the bootloader is also documented on the NixOS wiki here. The listed config worked for me, which is:

{ ... }: {
  boot.loader.grub = {
    efiSupport = true;
    efiInstallAsRemovable = true;
    device = "nodev";
  };
}

Notable things about the hardware UTM configures by default:

• Ethernet: enp0s5, with the host at 192.168.64.1/24 and the guest receiving an IP via DHCP (probably 192.168.64.2)
• Disk: root on vda2, boot on vda1

It's a QEMU VM so everything is virtio; NixOS provides a configuration to get all the necessary modules, which you can import at (modulesPath + "/profiles/qemu-guest.nix").

Here's the configuration I used:

{{ codefile(path="./configuration.nix", code_lang="nix", colocated=true, hide=true) }}

Build with:

$ nix-build -I "nixpkgs=channel:nixos-22.05,nixos-config=$(pwd)/configuration.nix" \
    '<nixpkgs>/nixos' -A config.system.build.image

Creating a VM

To do this, create a VM, selecting the "Other" type:

{% image(name="./1-os-dialog.png", colocated=true) %} Screenshot of the UTM operating system selection dialog, in which macOS, Linux, Windows, and Other are listed. {% end %}

Then disable ISO boot, since we don't need an installer where we're going.

{% image(name="./2-disable-iso-boot.png", colocated=true) %} Dialog in UTM: "disable ISO boot" checkbox checked {% end %}

Finish setting up the VM, don't worry about the disk size since we will immediately delete it. Select "Open VM Settings" on the summary page so you can do that.

{% image(name="./3-open-vm-settings.png", colocated=true) %} UTM VM creation summary dialog with "Open VM Settings" box checked {% end %}

Then select the disk and delete it:

{% image(name="./4-delete-volume.png", colocated=true) %} Screenshot of the VM settings showing the delete button of the disk selected in the sidebar. {% end %}

Finally, create a new disk, selecting import to get the newly built root filesystem image from Nix:

{% image(name="./5-import-image.png", colocated=true) %} Screenshot of the "create disk" dialog, with the import button visible. {% end %}