fix: use system.modulesTree for disk image VM kernel modules

[jctemp]

Problem

Building disk images with vmWithDisko fails on NixOS 25.11 with:

kernel-modules-shrunk> Required modules: 9p 9pnet_virtio virtiofs virtio_pci virtio_blk virtio_balloon virtio_rng
kernel-modules-shrunk> Can not derive a closure of kernel modules because no modules were provided.

Cause

The vmTools.override in lib/make-disk-images.nix was using:

kernel = pkgs.aggregateModules (
  [
    cfg.kernelPackages.kernel
  ]
  ++ lib.optional (cfg.kernelPackages.kernel ? modules) cfg.kernelPackages.kernel.modules
  ...
);

The issue: cfg.kernelPackages.kernel.modules points back to the kernel package itself (which contains only bzImage and System.map), not to the actual kernel modules package.

Further testing gave me this output:

$ nix eval '.#nixosConfigurations.test.config.boot.kernelPackages.kernel.modules.name'
"linux-6.12.59" 
$ find $(nix eval --raw '...kernel.modules.outPath') -name "*.ko*" | wc -l
0 

$ nix eval '.#nixosConfigurations.test.config.system.modulesTree.name'
"linux-6.12.59-modules" 
$ find $(nix eval --raw '...system.modulesTree.outPath') -name "*.ko*" | wc -l
7035 

To verify that the issue is due to version 25.11, I checked the 25.05 and 25.11 output to confirm that the kernel.modules attribute suddenly points to something.

# Test 25.05
nix eval --impure --expr 'let pkgs = (builtins.getFlake "github:nixos/nixpkgs/nixos-25.05").legacyPackages.x86_64-linux; in pkgs.linuxPackages.kernel.modules.name or "none"'

"none"

# Test 25.11
nix eval --impure --expr 'let pkgs = (builtins.getFlake "github:nixos/nixpkgs/nixos-25.11").legacyPackages.x86_64-linux; in pkgs.linuxPackages.kernel.modules.name or "none"'

"linux-6.12.59"

Solution

Use config.system.modulesTree instead, which correctly points to the linux-X.X.X-modules package containing the actual .ko kernel module files needed for the VM initrd.

Testing

• All x86_64-linux checks pass
• vmWithDisko now successfully builds and boots

Minimal Reproduction

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
    disko.url = "github:nix-community/disko/latest";
  };
  
  outputs = { nixpkgs, disko, ... }: {
    nixosConfigurations.test = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        disko.nixosModules.disko
        ({ modulesPath, ... }: {
          imports = [ (modulesPath + "/profiles/qemu-guest.nix") ];
          
          disko.devices.disk.main = {
            device = "/dev/vda";
            type = "disk";
            content = {
              type = "gpt";
              partitions = {
                esp = {
                  size = "512M";
                  type = "EF00";
                  content = {
                    type = "filesystem";
                    format = "vfat";
                    mountpoint = "/boot";
                  };
                };
                root = {
                  size = "100%";
                  content = {
                    type = "filesystem";
                    format = "ext4";
                    mountpoint = "/";
                  };
                };
              };
            };
          };
          
          boot.loader.systemd-boot.enable = true;
          system.stateVersion = "25.11";
        })
      ];
    };
  };
}

flake.lock

{ "nodes": { "disko": { "inputs": { "nixpkgs": "nixpkgs" }, "locked": { "lastModified": 1746728054, "narHash": "sha256-eDoSOhxGEm2PykZFa/x9QG5eTH0MJdiJ9aR00VAofXE=", "owner": "nix-community", "repo": "disko", "rev": "ff442f5d1425feb86344c028298548024f21256d", "type": "github" }, "original": { "owner": "nix-community", "ref": "latest", "repo": "disko", "type": "github" } }, "nixpkgs": { "locked": { "lastModified": 1746576598, "narHash": "sha256-FshoQvr6Aor5SnORVvh/ZdJ1Sa2U4ZrIMwKBX5k2wu0=", "owner": "NixOS", "repo": "nixpkgs", "rev": "b3582c75c7f21ce0b429898980eddbbf05c68e55", "type": "github" }, "original": { "owner": "NixOS", "ref": "nixpkgs-unstable", "repo": "nixpkgs", "type": "github" } }, "nixpkgs_2": { "locked": { "lastModified": 1764522689, "narHash": "sha256-SqUuBFjhl/kpDiVaKLQBoD8TLD+/cTUzzgVFoaHrkqY=", "owner": "nixos", "repo": "nixpkgs", "rev": "8bb5646e0bed5dbd3ab08c7a7cc15b75ab4e1d0f", "type": "github" }, "original": { "owner": "nixos", "ref": "nixos-25.11", "repo": "nixpkgs", "type": "github" } }, "root": { "inputs": { "disko": "disko", "nixpkgs": "nixpkgs_2" } } }, "root": "root", "version": 7 }

nix run '.#nixosConfigurations.test.config.system.build.vmWithDisko'
# Fails with "no modules were provided"

[jctemp]

Some random update on 25.11 has fixed the issue (don't know; I just updated my lock, and suddenly it works?). Still the issue remains: kernel.modules points to nothing in 25.05 and now it's suddenly the kernel itself. Whatever, the logic is broken, but it somehow works. Should I close the PR or keep it up?