feat: add vm-guest and vm-9p-automount modules

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
+    ];
+  };
+}