pkgs.writeErofsLayers,profiles/ch-runner: MVE

$ nix-build -A examples.dummy.config.debug.closure.erofs.list | xargs
cat | xargs du -h
749M    /nix/store/bzfv5x6lycq6hzhjv6d6vlk1q8fdg9di-base0.erofs
24M     /nix/store/hp41jfq36y0mmjrzqilyh3jfsvqic3kb-nixos.erofs

$ nix run -f . examples.dummy.config.uvms.cloud-hypervisor.runner
...
<<< Welcome to NixOS 25.11pre-git (x86_64) - ttyS0 >>>

nixos login:

The definition of the `pkgs` fixpoint was moved to pkgs/default.nix.
For that, dirToAttrs was moved to lib/, imported ad hoc

default.nix

@@ -1,26 +1,7 @@
 let
   pkgs = import <nixpkgs> { };
   inherit (pkgs) lib;
-  dirToAttrs =
-    root: patterns: f:
-    lib.listToAttrs (
-      lib.concatMap (
-        dirent:
-        let
-          fname = dirent.name;
-          typ = dirent.value;
-          fpath = root + "/${fname}";
-          doMatch =
-            pat:
-            let
-              match = pat fpath fname typ;
-              value = f match fpath typ;
-            in
-            if match == null then [ ] else [ (lib.nameValuePair match value) ];
-        in
-        (lib.take 1 (lib.concatMap (doMatch) patterns))
-      ) (lib.attrsToList (builtins.readDir root))
-    );
+  dirToAttrs = import ./lib/dirToAttrs.nix { inherit lib; };
 in
 {
   examples =
@@ -35,23 +16,5 @@ in
         name: fpath: _:
         import <nixpkgs/nixos/lib/eval-config.nix> { modules = [ fpath ]; }
       );
-  pkgs = lib.makeScope pkgs.newScope (
-    self:
-    dirToAttrs ./pkgs
-      [
-        (
-          path: fname: _:
-          lib.strings.removeSuffix ".nix" fname
-        )
-      ]
-      (
-        name: fpath: typ:
-        if typ == "regular" then
-          self.callPackage fpath { }
-        else if typ == "directory" && builtins.pathExists (fpath + "/package.nix") then
-          self.callPackage (fpath + "/package.nix") { }
-        else
-          null
-      )
-  );
+  pkgs = pkgs.callPackage ./pkgs { };
 }

examples/dummy.nix

@@ -2,21 +2,37 @@
   config,
   lib,
   pkgs,
+  modulesPath,
   ...
 }:
 {
   imports = [
     ../profiles/all.nix
+    (modulesPath + "/profiles/minimal.nix")
   ];
 
+  system.stateVersion = "25.11";
+
   # following microvm.nix:
   boot.loader.grub.enable = false;
+  boot.initrd.systemd.enable = true;
   fileSystems."/" = lib.mkDefault {
     device = "rootfs"; # how does this work? does this assign a label to the tmpfs?
     fsType = "tmpfs";
-    options = [ "size=10%,mode=0755" ];
+    options = [ "size=20%,mode=0755" ];
     neededForBoot = true;
   };
+  boot.initrd.systemd.emergencyAccess = true;
+  boot.initrd.systemd.settings.Manager.DefaultTimeoutStartSec = 15;
+  systemd.settings.Manager.DefaultTimeoutStartSec = 15;
+  networking.useNetworkd = true;
+  networking.nftables.enable = true;
+
+  uvms.cloud-hypervisor.enable = true;
+
+  users.mutableUsers = false;
+  users.users.root.password = "hacktheplanet!";
+
 
   # TODO: cmdline, kernel, initrd, fileSystems
 }

lib/dirToAttrs.nix

@@ -0,0 +1,20 @@
+{ lib }:
+root: patterns: f:
+lib.listToAttrs (
+  lib.concatMap (
+    dirent:
+    let
+      fname = dirent.name;
+      typ = dirent.value;
+      fpath = root + "/${fname}";
+      doMatch =
+        pat:
+        let
+          match = pat fpath fname typ;
+          value = f match fpath typ;
+        in
+        if match == null then [ ] else [ (lib.nameValuePair match value) ];
+    in
+    (lib.take 1 (lib.concatMap (doMatch) patterns))
+  ) (lib.attrsToList (builtins.readDir root))
+)

lib/mkIfMicrovmGuest.nix

@@ -0,0 +1,7 @@
+{
+  lib,
+  options,
+  config,
+}:
+def:
+lib.optionalAttrs (options ? "microvm"."guest"."enable") (lib.mkIf config.microvm.guest.enable def)

pkgs/default.nix

@@ -0,0 +1,23 @@
+{ lib, newScope }:
+let
+  dirToAttrs = import ../lib/dirToAttrs.nix { inherit lib; };
+in
+lib.makeScope newScope (
+  self:
+  dirToAttrs ./.
+    [
+      (
+        path: fname: _:
+        if fname == "default.nix" then null else lib.strings.removeSuffix ".nix" fname
+      )
+    ]
+    (
+      name: fpath: typ:
+      if typ == "regular" then
+        self.callPackage fpath { }
+      else if typ == "directory" && builtins.pathExists (fpath + "/package.nix") then
+        self.callPackage (fpath + "/package.nix") { }
+      else
+        null
+    )
+)

pkgs/writeErofs/package.nix

@@ -0,0 +1,84 @@
+{
+  lib,
+  erofs-utils,
+  runCommand,
+  python3Minimal,
+  bubblewrap,
+  writeClosure,
+}:
+{
+  name ? "${label}.erofs",
+  label ? "image",
+  roots ? [ ],
+  rootsExclude ? [ ],
+  erofsArgs ? [
+    "--mount-point=/nix/store"
+    "-z"
+    "lz4hc,level=3"
+    "-T0"
+    "-L"
+    label
+    "--all-root"
+  ],
+  ...
+}@attrs:
+
+let
+  old = writeClosure rootsExclude;
+  new = writeClosure roots;
+  attrs' = lib.removeAttrs [ "name" "roots" "rootsExclude" "erofsArgs" ] attrs;
+in
+runCommand name
+  (
+    attrs
+    // {
+      __structuredAttrs = true;
+      inherit name label;
+      unsafeDiscardReferences.out = true;
+      nativeBuildInputs = [
+        erofs-utils
+        bubblewrap
+        python3Minimal
+      ]
+      ++ attrs.nativeBuildInputs or [ ];
+      inherit old new erofsArgs;
+    }
+  )
+  ''
+    mkdir -p store
+    python3 << EOF
+    import hashlib, json, os, subprocess
+    from uuid import UUID
+
+
+    def uuid_from_node(node: int = 0) -> UUID:
+        return UUID(fields=(*((0,) * 5), node))
+
+
+    out_path = os.environ.get("out")
+    default_uuid = uuid_from_node((2**48 - 1) & int.from_bytes(hashlib.sha256(out_path.encode("ascii")).digest()))
+    with open(os.environ.get("NIX_ATTRS_JSON_FILE"), mode="r") as f:
+         attrs = json.load(f)
+    name = attrs["name"]
+    with open(attrs["old"], mode="r") as f:
+        old_paths = set(ln.rstrip() for ln in f)
+    with open(attrs["new"], mode="r") as f:
+        new_paths = [ln.rstrip() for ln in f]
+    new_paths = [p for p in new_paths if p not in old_paths]
+    erofs_args = attrs.get("erofsArgs", [])
+    if not any(a.startswith("-U") for a in erofs_args):
+        erofs_args.extend(["-U", str(default_uuid)])
+    subprocess.run(
+      check=True,
+      env=os.environ,
+      args=[
+          "bwrap",
+          "--dev-bind", "/", "/",
+          "--chdir", os.getcwd(),
+          *[a for p in new_paths for a in [f"--ro-bind", p, f"{os.getcwd()}/store/{os.path.basename(p)}"]],
+          "--",
+          "mkfs.erofs", *attrs.get("erofsArgs", []), out_path, "store",
+      ],
+    )
+    EOF
+  ''

pkgs/writeErofsLayers/package.nix

@@ -0,0 +1,35 @@
+{ lib, writeErofs }:
+{
+  layers,
+  labelLower ? "base",
+  labelUpper ? "image",
+}:
+
+let
+  inherit (lib.lists) toList;
+  nLayers = builtins.length layers;
+  mkLabel = seq: if seq + 1 < nLayers then labelLower + toString seq else labelUpper;
+  f =
+    {
+      roots ? [ ],
+      layers ? [ ],
+      seq ? 0,
+    }:
+    newRoots: {
+      seq = seq + 1;
+      roots = roots ++ (toList newRoots);
+      layers = layers ++ [
+        (writeErofs ({
+          roots = toList newRoots;
+          rootsExclude = roots;
+          passthru = {
+            inherit seq;
+            label = mkLabel seq;
+          };
+          label = mkLabel seq;
+        }))
+      ];
+    };
+  acc = lib.foldl f { } layers;
+in
+acc.layers

profiles/ch-runner.nix

@@ -0,0 +1,114 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+# It is not the intent to stick to the microvm.nix-like static interface,
+# but we shall begin by reproducing at least some of their work.
+
+let
+  cfg = config.uvms.cloud-hypervisor;
+  inherit (config.debug.closure.erofs) layers;
+in
+{
+  options = {
+    uvms.cloud-hypervisor.enable = lib.mkEnableOption "Configure guest (e.g. fileSystems)";
+    uvms.cloud-hypervisor.runner = lib.mkOption {
+      type = lib.types.package;
+      description = "A naive script for running this system in cloud-hypervisor";
+    };
+    uvms.cloud-hypervisor.extraArgv = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+    };
+    uvms.cloud-hypervisor.argv = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+    };
+    uvms.cloud-hypervisor.extraCmdline = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+    };
+    uvms.cloud-hypervisor.cmdline = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [
+        "earlyprintk=ttyS0"
+        "console=ttyS0"
+        "reboot=t"
+        "panic=-1"
+        "init=${config.system.build.toplevel}/init"
+      ]
+      ++ config.boot.kernelParams
+      ++ config.uvms.cloud-hypervisor.extraCmdline;
+    };
+  };
+  config = lib.mkMerge [
+    {
+      uvms.cloud-hypervisor.argv = lib.mkBefore (
+        [
+          (lib.getExe pkgs.cloud-hypervisor)
+          "--cmdline=${lib.concatStringsSep " " cfg.cmdline}"
+          "--kernel=${config.boot.kernelPackages.kernel}/${pkgs.stdenv.hostPlatform.linux-kernel.target}"
+          "--initramfs=${config.system.build.initialRamdisk}/${config.system.boot.loader.initrdFile}"
+          "--vsock=cid=4,socket=CONNECT.sock"
+          "--api-socket=ch.sock"
+          "--serial=tty"
+          "--console=null"
+          "--watchdog"
+          "--seccomp=true"
+        ]
+        ++ cfg.extraArgv
+      );
+      uvms.cloud-hypervisor.runner = pkgs.writeShellScriptBin "run-${config.networking.hostName}" ''
+        set -euo pipefail
+        GUESTNAME=${config.networking.hostName}
+        args=(
+        ${lib.concatMapStringsSep "\n" lib.escapeShellArg cfg.argv}
+        )
+        mkdir -p "$HOME/uvms/$GUESTNAME"
+        cd "$HOME/uvms/$GUESTNAME"
+        cleanup() {
+            rm "$HOME/uvms/$GUESTNAME"/{ch,CONNECT}.sock
+        }
+        exec -a "uuvm/$GUESTNAME" "''${args[@]}"
+      '';
+    }
+    (lib.mkIf cfg.enable {
+      boot.initrd.availableKernelModules = [
+        "erofs"
+        "overlay"
+        "virtio_mmio"
+        "virtio_pci"
+        "virtio_blk"
+        # "9pnet_virtio"
+        # "9p"
+        "virtiofs"
+      ];
+      boot.initrd.systemd.enable = true;
+      fileSystems = {
+        "/nix/store" = {
+          fsType = "overlay";
+          overlay.lowerdir = map (img: "/nix/.ro-stores/${toString img.seq}") layers;
+          neededForBoot = true;
+        };
+      }
+      // lib.listToAttrs (
+        map (
+          img:
+          lib.nameValuePair "/nix/.ro-stores/${toString img.seq}" {
+            device = "/dev/disk/by-label/${img.label}";
+            neededForBoot = true;
+            options = [ "x-systemd.device-timeout=5" ];
+          }
+        ) layers
+      );
+      uvms.cloud-hypervisor.argv = [
+        "--memory=size=1536M,hotplug_size=1536M,hotplugged_size=512M,hotplug_method=virtio-mem,mergeable=on,shared=on"
+        "--cpus=boot=4"
+        "--disk"
+      ]
+      ++ map (img: "path=${img},readonly=true,id=${toString img.label}") layers;
+    })
+  ];
+}

profiles/debug-closure.nix

@@ -0,0 +1,85 @@
+{
+  config,
+  lib,
+  pkgs,
+  modulesPath,
+  ...
+}:
+
+let
+  cfg = config.debug.closure;
+  inherit (lib.types) package;
+  inherit (config.system.build) toplevel;
+
+  ps = pkgs.callPackage ../pkgs { };
+  inherit (ps) writeErofsLayers;
+  emptySystem = import (pkgs.path + "/nixos/lib/eval-config.nix") {
+    modules = [
+      (modulesPath + "/profiles/minimal.nix")
+      {
+        system.stateVersion = config.system.stateVersion;
+        fileSystems."/".fsType = "tmpfs";
+        boot.loader.grub.enable = false;
+        networking.hostName = "base";
+        networking.nftables.enable = true;
+        networking.useNetworkd = true;
+        systemd.network.enable = true;
+      }
+    ];
+  };
+in
+{
+  options.debug.closure = {
+    info = lib.mkOption {
+      type = package;
+      description = "closureInfo { rootPaths = [ toplevel ]; }";
+    };
+    write = lib.mkOption {
+      type = package;
+      description = "writeClosure [ toplevel ]";
+    };
+    extraLayers = lib.mkOption {
+      type = lib.types.listOf package;
+      description = "Roots from which to generate `erofs.layers`";
+      defaultText = ''
+        [
+          (nixosSystem { modules = [ ]; }).config.system.build.toplevel
+          # implicitly appended: config.system.build.toplevel
+        ]
+      '';
+    };
+    erofs.layers = lib.mkOption {
+      type = lib.types.listOf package;
+      description = "writeErofs [ ... ]";
+      readOnly = true;
+    };
+    erofs.list = lib.mkOption {
+      type = package;
+      description = "writeClosure [ (writeErofs [ ... ])... ]";
+    };
+  };
+  config.debug.closure = {
+    info = pkgs.closureInfo { rootPaths = [ toplevel ]; };
+    write = pkgs.writeClosure [ toplevel ];
+
+    erofs.list =
+      pkgs.runCommand "layers"
+        {
+          __structuredAttrs = true;
+          inherit (cfg.erofs) layers;
+        }
+        ''
+          printf "%s\n" "''${layers[@]}" > "$out"
+        '';
+    extraLayers = lib.mkMerge [
+      (lib.mkBefore [ emptySystem.config.system.build.toplevel ])
+      # (lib.mkAfter [config.system.build.toplevel])
+    ];
+    erofs.layers = writeErofsLayers {
+      layers = cfg.extraLayers ++ [
+        toplevel
+      ];
+      labelUpper = config.networking.hostName;
+    };
+  };
+}

profiles/resources.nix

@@ -6,7 +6,7 @@
       # zswap is said to be more reliable than zram
       boot.kernelParams = lib.optionals (!config.zramSwap.enable) [ "zswap.enabled=1" ];
     }
-    (lib.optionalAttrs (options ? microvm) {
+    (lib.optionalAttrs (options ? "microvm" && config.microvm.guest.enable) {
       microvm = {
         hypervisor = lib.mkDefault "cloud-hypervisor";
         graphics.enable = lib.mkDefault true;

profiles/uvms-guest.nix

@@ -1,47 +1,42 @@
 {
   options,
+  config,
   lib,
   pkgs,
   ...
 }:
+let
+  mkIfGuest = import ../lib/mkIfMicrovmGuest.nix { inherit options config lib; };
+in
 {
   imports = [
     ./vsock-connect-guest.nix
     ./uvms-users.nix
   ];
-  config = lib.optionalAttrs (options ? microvm) {
-    microvm = {
-      hypervisor = "cloud-hypervisor";
-      volumes = [
+  config = lib.mkMerge [
+    (mkIfGuest {
+      microvm = {
+        hypervisor = "cloud-hypervisor";
+        volumes = [
+          {
+            image = "swapfile.img";
+            mountPoint = "/var/swapfiles";
+            size = 1024;
+          }
+        ];
+      };
+      swapDevices = [
         {
-          image = "swapfile.img";
-          mountPoint = "/var/swapfiles";
-          size = 1024;
+          device = "/var/swapfiles/swap0";
+          size = 768;
         }
       ];
-      cloud-hypervisor.extraArgs = [
-        "--api-socket=ch.sock"
-      ];
-    };
-    swapDevices = [
-      {
-        device = "/var/swapfiles/swap0";
-        size = 768;
-      }
-    ];
+    })
+    {
 
-    boot.kernelParams = [ "zswap.enabled=1" ];
-    zramSwap.enable = false;
+      boot.kernelParams = [ "zswap.enabled=1" ];
+      zramSwap.enable = false;
 
-    systemd.services."microvm@".serviceConfig.ExecStartPost =
-      pkgs.writeShellScript "microvm-fix-umask" ''
-        if [[ -e CONNECT.vsock ]] ; then
-          chmod g+r CONNECT.vsock
-        fi
-      '';
-
-    systemd.tmpfiles.settings."10-muvm" = {
-      "/var/lib/microvms/*/CONNECT.sock".z.mode = "660";
-    };
-  };
+    }
+  ];
 }

profiles/vsock-connect-guest.nix

@@ -1,18 +1,27 @@
-{ options, lib, ... }:
 {
-  config =
-    lib.optionalAttrs (options ? "microvm") {
+  options,
+  config,
+  lib,
+  ...
+}:
+let
+  mkIfGuest = import ../lib/mkIfMicrovmGuest.nix { inherit options config lib; };
+in
+{
+  config = lib.mkMerge [
+    (mkIfGuest {
       microvm.cloud-hypervisor.extraArgs = [
         "--vsock"
         "cid=4,socket=CONNECT.sock"
       ];
-    }
-    // {
+    })
+    {
       # Somehow, sshd calls to PAM with PAM_RHOST="UNKNOWN",
       # prompting a slow DNS look-up each time...
       #
       # https://mastodon.acm.org/@nobody/115108458851355328
       # https://github.com/linux-pam/linux-pam/issues/885#issuecomment-3030698895
       networking.hosts."100::" = [ "UNKNOWN" ];
-    };
+    }
+  ];
 }