merge: ephemeral vm setup

flake.lock

@@ -468,6 +468,22 @@
         "type": "github"
       }
     },
+    "nvim": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1771940146,
+        "narHash": "sha256-z52fdPOWd3hUKKd9IojZ6rFYAgQrdrw/lNKFVdO2jf8=",
+        "ref": "refs/heads/main",
+        "rev": "058c9c730d8d60f482d97b61702561e1ea978847",
+        "revCount": 91,
+        "type": "git",
+        "url": "https://git.janezic.dev/janezicmatej/nvim.git"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://git.janezic.dev/janezicmatej/nvim.git"
+      }
+    },
     "pre-commit": {
       "inputs": {
         "flake-compat": "flake-compat",
@@ -503,6 +519,7 @@
         "nixpkgs": "nixpkgs_3",
         "nixpkgs-master": "nixpkgs-master",
         "nixpkgs-unstable": "nixpkgs-unstable",
+        "nvim": "nvim",
         "stylix": "stylix"
       }
     },

flake.nix

@@ -10,10 +10,10 @@
     #   url = "git+https://git.janezic.dev/janezicmatej/.dotfiles.git";
     #   flake = false;
     # };
-    # nvim = {
-    #   url = "git+https://git.janezic.dev/janezicmatej/nvim.git?ref=rewrite";
-    #   flake = false;
-    # };
+    nvim = {
+      url = "git+https://git.janezic.dev/janezicmatej/nvim.git";
+      flake = false;
+    };
 
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
 
@@ -82,6 +82,11 @@
         iso = mkHost "iso" {
           system = "x86_64-linux";
         };
+
+        ephvm = mkHost "ephvm" {
+          system = "x86_64-linux";
+          user = "matej";
+        };
       };
 
       nixosModules = import ./modules/nixos {
@@ -123,6 +128,7 @@
           packages = [
             pkgs.pre-commit
             pkgs.statix
+            pkgs.qemu
           ];
         };
       }

hosts/ephvm/configuration.nix

@@ -0,0 +1,69 @@
+{
+  pkgs,
+  lib,
+  inputs,
+  config,
+  ...
+}:
+{
+  networking.hostName = "ephvm";
+
+  profiles.base.enable = true;
+
+  vm-guest = {
+    enable = true;
+    headless = true;
+  };
+
+  vm-9p-automount = {
+    enable = true;
+    user = "matej";
+  };
+
+  localisation = {
+    timeZone = "UTC";
+    defaultLocale = "en_US.UTF-8";
+  };
+
+  virtualisation.docker = {
+    enable = true;
+    logDriver = "json-file";
+  };
+
+  # TODO:(@janezicmatej) move neovim dotfiles wiring to a cleaner place
+  home-manager.users.matej = {
+    neovim.dotfiles = inputs.nvim;
+  };
+
+  # writable claude config via 9p
+  fileSystems."/home/matej/.claude" = {
+    device = "claude";
+    fsType = "9p";
+    options = [
+      "trans=virtio"
+      "version=9p2000.L"
+      "nofail"
+      "x-systemd.automount"
+    ];
+  };
+
+  # .claude.json passed via qemu fw_cfg
+  boot.kernelModules = [ "qemu_fw_cfg" ];
+  systemd.services.claude-json = {
+    after = [ "systemd-modules-load.service" ];
+    wants = [ "systemd-modules-load.service" ];
+    wantedBy = [ "multi-user.target" ];
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true;
+      ExecStart = pkgs.writeShellScript "claude-json" ''
+        src="/sys/firmware/qemu_fw_cfg/by_name/opt/claude.json/raw"
+        [ -f "$src" ] || exit 0
+        cp "$src" /home/matej/.claude.json
+        chown matej:users /home/matej/.claude.json
+      '';
+    };
+  };
+
+  system.stateVersion = "25.11";
+}

hosts/ephvm/hardware-configuration.nix

@@ -0,0 +1,20 @@
+{
+  lib,
+  pkgs,
+  modulesPath,
+  ...
+}:
+{
+  imports = [
+    (modulesPath + "/profiles/qemu-guest.nix")
+  ];
+
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/nixos";
+    autoResize = true;
+    fsType = "ext4";
+  };
+
+  # image.modules (disk-image.nix) overrides boot loader per variant
+  boot.loader.grub.device = lib.mkDefault "/dev/vda";
+}

justfile

@@ -33,6 +33,18 @@ build:
 iso:
     nixos-rebuild build-image --image-variant iso-installer --flake .#iso
 
+# build ephemeral VM image
+ephvm-build:
+    nixos-rebuild build-image --image-variant qemu --flake .#ephvm
+
+# run ephemeral VM
+ephvm-run *ARGS:
+    bash scripts/ephvm-run.sh $(find -L result -name '*.qcow2' | head -1) {{ARGS}}
+
+# ssh into running ephemeral VM
+ephvm-ssh port="2222":
+    ssh -p {{port}} -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null matej@localhost
+
 # garbage collect old generations
 clean:
     sudo nix-collect-garbage $(nix eval --raw -f ./nix.nix nix.gc.options)

modules/nixos/vm-9p-automount.nix

@@ -0,0 +1,74 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+let
+  inherit (config.vm-9p-automount) user;
+  inherit (config.users.users.${user}) home group;
+in
+{
+  options = {
+    vm-9p-automount = {
+      enable = lib.mkEnableOption "auto-discover and mount 9p shares";
+
+      user = lib.mkOption {
+        type = lib.types.str;
+        description = "user to own the mount points";
+      };
+
+      prefix = lib.mkOption {
+        type = lib.types.str;
+        default = "mount_";
+        description = "9p mount tag prefix to match";
+      };
+
+      basePath = lib.mkOption {
+        type = lib.types.str;
+        default = "${home}/mnt";
+        description = "directory to mount shares under";
+      };
+    };
+  };
+
+  config = lib.mkIf config.vm-9p-automount.enable {
+    systemd.services.vm-9p-automount = {
+      description = "Auto-discover and mount 9p shares";
+      after = [
+        "local-fs.target"
+        "nss-user-lookup.target"
+        "systemd-modules-load.service"
+      ];
+      wants = [ "systemd-modules-load.service" ];
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = pkgs.writeShellScript "vm-9p-automount" ''
+          BASE="${config.vm-9p-automount.basePath}"
+          PREFIX="${config.vm-9p-automount.prefix}"
+          mkdir -p "$BASE"
+          chown ${user}:${group} "$BASE"
+
+          for tagfile in $(find /sys/devices -name mount_tag 2>/dev/null); do
+            [ -f "$tagfile" ] || continue
+            tag=$(tr -d '\0' < "$tagfile")
+
+            case "$tag" in
+              "$PREFIX"*) ;;
+              *) continue ;;
+            esac
+
+            name="''${tag#"$PREFIX"}"
+            target="$BASE/$name"
+
+            mkdir -p "$target"
+            ${pkgs.util-linux}/bin/mount -t 9p "$tag" "$target" \
+              -o trans=virtio,version=9p2000.L || continue
+          done
+        '';
+      };
+    };
+  };
+}

modules/nixos/vm-guest.nix

@@ -0,0 +1,49 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+{
+  options = {
+    vm-guest = {
+      enable = lib.mkEnableOption "VM guest configuration";
+      headless = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = "run without display, serial console only";
+      };
+    };
+  };
+
+  config = lib.mkIf config.vm-guest.enable {
+    services.qemuGuest.enable = true;
+    services.spice-vdagentd.enable = lib.mkIf (!config.vm-guest.headless) true;
+
+    boot.kernelParams = lib.mkIf config.vm-guest.headless [ "console=ttyS0,115200" ];
+
+    # 9p for host file mounting
+    boot.initrd.availableKernelModules = [
+      "9p"
+      "9pnet_virtio"
+    ];
+    boot.kernelModules = [
+      "9p"
+      "9pnet_virtio"
+    ];
+
+    networking = {
+      useDHCP = true;
+      firewall.allowedTCPPorts = [ 22 ];
+    };
+
+    security.sudo.wheelNeedsPassword = false;
+
+    environment.systemPackages = with pkgs; [
+      curl
+      wget
+      htop
+      sshfs
+    ];
+  };
+}

scripts/ephvm-run.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SSH_PORT=2222
+MEMORY=8G
+CPUS=4
+MOUNTS=()
+CLAUDE_DIR=""
+CLAUDE_JSON=""
+IMAGE=""
+
+usage() {
+  cat <<EOF
+Usage: ephvm-run.sh <image.qcow2> [options]
+
+Options:
+  --mount <path>       Mount host directory into VM (repeatable)
+  --claude <path>      Mount claude config dir writable into VM
+  --claude-json <path> Copy claude.json into mounted claude dir
+  --memory <size>      VM memory (default: 8G)
+  --cpus <n>         VM CPUs (default: 4)
+  --ssh-port <port>  SSH port forward (default: 2222)
+EOF
+  exit 1
+}
+
+[ "${1:-}" ] || usage
+
+IMAGE="$1"
+shift
+
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --mount)      MOUNTS+=("$2"); shift 2 ;;
+    --claude)     CLAUDE_DIR="$2"; shift 2 ;;
+    --claude-json) CLAUDE_JSON="$2"; shift 2 ;;
+    --memory)     MEMORY="$2"; shift 2 ;;
+    --cpus)     CPUS="$2"; shift 2 ;;
+    --ssh-port) SSH_PORT="$2"; shift 2 ;;
+    *)          echo "unknown option: $1"; usage ;;
+  esac
+done
+
+if [ ! -f "$IMAGE" ]; then
+  echo "error: image not found: $IMAGE"
+  exit 1
+fi
+
+ACCEL="tcg"
+[ -r /dev/kvm ] && ACCEL="kvm"
+
+QEMU_ARGS=(
+  qemu-system-x86_64
+  -accel "$ACCEL"
+  -m "$MEMORY"
+  -smp "$CPUS"
+  -drive "file=$IMAGE,format=qcow2,snapshot=on"
+  -nic "user,hostfwd=tcp::${SSH_PORT}-:22"
+  -nographic
+)
+
+if [ "$ACCEL" != "tcg" ]; then
+  QEMU_ARGS+=(-cpu host)
+fi
+
+FS_ID=0
+for mount_path in "${MOUNTS[@]}"; do
+  mount_path=$(realpath "$mount_path")
+  name=$(basename "$mount_path")
+  QEMU_ARGS+=(
+    -virtfs "local,path=$mount_path,mount_tag=mount_$name,security_model=none,id=fs${FS_ID}"
+  )
+  FS_ID=$((FS_ID + 1))
+done
+
+if [ -n "$CLAUDE_DIR" ]; then
+  CLAUDE_DIR=$(realpath "$CLAUDE_DIR")
+  QEMU_ARGS+=(
+    -virtfs "local,path=$CLAUDE_DIR,mount_tag=claude,security_model=none,id=fs${FS_ID}"
+  )
+fi
+
+if [ -n "$CLAUDE_JSON" ]; then
+  QEMU_ARGS+=(-fw_cfg "name=opt/claude.json,file=$CLAUDE_JSON")
+fi
+
+exec "${QEMU_ARGS[@]}"