nixos: turn vmWithBootLoader into option (nixos-rebuild build-vm)

[roberth]

Motivation for this change

• Clean up by not repeating evalModules calls or vm definitions.
• Allow users to affect the nixos-rebuild build-vm configuration directly, by setting configs in the new options.
• Make the vm and vmWithBootloader attrs less special. These are now in regular config values.
• Fix the type of system.build.

This is similar to #144094

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[ncfavier]

I think nixos-rebuild would have to be updated to handle build-vm* with flakes correctly.

[ncfavier]

Looks great now!

[ncfavier]

Maybe we can add a boot.isVM option, in the spirit of boot.isContainer?

[roberth]

Maybe we can add a boot.isVM option, in the spirit of boot.isContainer?

Not a fan of such options, particularly when they're intended to remove things after the fact. The current solution is already more in the spirit of #148456 so I would consider depending on it to be somewhat of a regression. That said, I do think a use case may arise, but I'm not eager to add this option until we know for sure that we need it.

[roberth]

Looks great now!

Thanks for the fast paced collaboration / review!

[ncfavier]

Not a fan of such options, particularly when they're intended to remove things after the fact. The current solution is already more in the spirit of #148456 so I would consider depending on it to be somewhat of a regression. That said, I do think a use case may arise, but I'm not eager to add this option until we know for sure that we need it.

OK!

Looks great now!

Thanks for the fast paced collaboration / review!

NP!

[dasJ]

Is this ready to get merged?

[ncfavier]

I think so.

[ncfavier]

I've tested that nixos-rebuild build-vm and build-vm-with-bootloader work fine both with flakes and non-flakes. This is OK to merge.

[ncfavier]

This conflicts with #149532 in some way (tested before/after) and causes the manual to fail to evaluate.

Cacheable portion of option doc build failed.
Usually this means that an option attribute that ends up in documentation (eg default or description) depends on the restricted module arguments config or pkgs.

[roberth]

Created #155022 but can't merge in the coming ~2h :( g2g

[Atemu]

I greatly appreciate the change and am in favor of its spirit but the fallout needs to be reduced IMO.

This broke my VM detection and made my regular configs have VM settings. This could have devastating effects and implications.

builtins.hasAttr "vm" config.system.build

I'm not sure what other mechanisms there were but I'd prefer they stayed in-tact (with a deprecation notice) and only broke once the new mechanism has reached a stable release. Is that possible?

[roberth]

Perhaps the attributes could be renamed, so that they don't trigger such logic.

I'm not sure what other mechanisms there were

None that I know of and none documented here in the manual at least https://nixos.org/manual/nixos/stable/#sec-changing-config

[ncfavier]

I am in favour of renaming the attributes, perhaps to something like virtualisation.build.vm* ?

[Atemu]

This is also very non-trivial/near impossible to work around as if then else doesn't play well with the module system and an ugly mkMerge of mkIfs still fails because virtualisation.vmVariant (guarded behind a false mkIf statement!) doesn't exist in older Nixpkgs and the module system still complains.

I finally found a workaround (_module.check = false; and virtualisation.vmVariant = lib.mkIf ...) but that took quite a while to debug.

[ElvishJerricco]

Has this broken the virtualisation options in qemu-vm.nix? I can't set e.g. virtualisation.cores = 4; and use nix-build ./nixpkgs/nixos --arg configuration ./configuration.nix -A vm. I get error: The option `virtualisation.cores' does not exist.

EDIT: Oh I see, I had to add imports = [./nixos/modules/virtualisation/qemu-vm.nix];

[jackinloadup]

I'm running into the same issue as @ElvishJerricco but either I don't understand the proposed solution or it only works within nixpkgs. A related issue #59219 proposes a similar solution but is impure with <nixpkgs>. Is this import required by design? It seems like this file should already be imported at

nixpkgs/nixos/modules/virtualisation/build-vm.nix

Lines 8 to 10 in 55c742c

	vmVariant = extendModules {
	modules = [ ./qemu-vm.nix ];
	};

as part of nixos-rebuild build-vm.

I could be misunderstanding as I'm still a nix beginner. My goal is to test my nixosConfigurations locally while tweaking them so I don't need to be near the target machine to see changes to the boot process.

[roberth]

@jackinloadup I think ElvishJerricco's problem was unrelated to this PR.

nixos-rebuild build-vm should just work. If your configuration isn't quite compatible with the default vm variant of your configuration, you may have to add some overrides in the virtualisation.vmVariant option. It takes any NixOS option; for example: virtualisation.vmVariant.services.nginx.enable = lib.mkForce false;

If you know of a realistic option that needs to be overridden for nixos-rebuild build-vm, it'd be great to add it to the manual.