qemu-vm.nix

# This module creates a virtual machine from the NixOS configuration.
# Building the `config.system.build.vm' attribute gives you a command
# that starts a KVM/QEMU VM running the NixOS configuration defined in
# `config'.  The Nix store is shared read-only with the host, which
# makes (re)building VMs very efficient.  However, it also means you
# can't reconfigure the guest inside the guest - you need to rebuild
# the VM in the host.  On the other hand, the root filesystem is a
# read/writable disk image persistent across VM reboots.

{ config, lib, pkgs, options, ... }:

with lib;

let

  qemu-common = import ../../lib/qemu-common.nix { inherit lib pkgs; };

  cfg = config.virtualisation;

  opt = options.virtualisation;

  qemu = cfg.qemu.package;

  consoles = lib.concatMapStringsSep " " (c: "console=${c}") cfg.qemu.consoles;

  driveOpts = { ... }: {

    options = {

      file = mkOption {
        type = types.str;
        description = lib.mdDoc "The file image used for this drive.";
      };

      driveExtraOpts = mkOption {
        type = types.attrsOf types.str;
        default = {};
        description = lib.mdDoc "Extra options passed to drive flag.";
      };

      deviceExtraOpts = mkOption {
        type = types.attrsOf types.str;
        default = {};
        description = lib.mdDoc "Extra options passed to device flag.";
      };

      name = mkOption {
        type = types.nullOr types.str;
        default = null;
        description =
          lib.mdDoc "A name for the drive. Must be unique in the drives list. Not passed to qemu.";
      };

    };

  };

  selectPartitionTableLayout = { useEFIBoot, useDefaultFilesystems }:
  if useDefaultFilesystems then
    if useEFIBoot then "efi" else "legacy"
  else "none";

  driveCmdline = idx: { file, driveExtraOpts, deviceExtraOpts, ... }:
    let
      drvId = "drive${toString idx}";
      mkKeyValue = generators.mkKeyValueDefault {} "=";
      mkOpts = opts: concatStringsSep "," (mapAttrsToList mkKeyValue opts);
      driveOpts = mkOpts (driveExtraOpts // {
        index = idx;
        id = drvId;
        "if" = "none";
        inherit file;
      });
      deviceOpts = mkOpts (deviceExtraOpts // {
        drive = drvId;
      });
      device =
        if cfg.qemu.diskInterface == "scsi" then
          "-device lsi53c895a -device scsi-hd,${deviceOpts}"
        else
          "-device virtio-blk-pci,${deviceOpts}";
    in
      "-drive ${driveOpts} ${device}";

  drivesCmdLine = drives: concatStringsSep "\\\n    " (imap1 driveCmdline drives);


  # Creates a device name from a 1-based a numerical index, e.g.
  # * `driveDeviceName 1` -> `/dev/vda`
  # * `driveDeviceName 2` -> `/dev/vdb`
  driveDeviceName = idx:
    let letter = elemAt lowerChars (idx - 1);
    in if cfg.qemu.diskInterface == "scsi" then
      "/dev/sd${letter}"
    else
      "/dev/vd${letter}";

  lookupDriveDeviceName = driveName: driveList:
    (findSingle (drive: drive.name == driveName)
      (throw "Drive ${driveName} not found")
      (throw "Multiple drives named ${driveName}") driveList).device;

  addDeviceNames =
    imap1 (idx: drive: drive // { device = driveDeviceName idx; });

  # Shell script to start the VM.
  startVM =
    ''
      #! ${cfg.host.pkgs.runtimeShell}

      export PATH=${makeBinPath [ cfg.host.pkgs.coreutils ]}''${PATH:+:}$PATH

      set -e

      NIX_DISK_IMAGE=$(readlink -f "''${NIX_DISK_IMAGE:-${toString config.virtualisation.diskImage}}") || test -z "$NIX_DISK_IMAGE"

      if test -n "$NIX_DISK_IMAGE" && ! test -e "$NIX_DISK_IMAGE"; then
          echo "Disk image do not exist, creating the virtualisation disk image..."
          # If we are using a bootloader and default filesystems layout.
          # We have to reuse the system image layout as a backing image format (CoW)
          # So we can write on the top of it.

          # If we are not using the default FS layout, potentially, we are interested into
          # performing operations in postDeviceCommands or at early boot on the raw device.
          # We can still boot through QEMU direct kernel boot feature.

          # CoW prevent size to be attributed to an image.
          # FIXME: raise this issue to upstream.
          ${qemu}/bin/qemu-img create \
          ${concatStringsSep " \\\n" ([ "-f qcow2" ]
          ++ optional (cfg.useBootLoader && cfg.useDefaultFilesystems) "-F qcow2 -b ${systemImage}/nixos.qcow2"
          ++ optional (!(cfg.useBootLoader && cfg.useDefaultFilesystems)) "-o size=${toString config.virtualisation.diskSize}M"
          ++ [ ''"$NIX_DISK_IMAGE"'' ])}
          echo "Virtualisation disk image created."
      fi

      # Create a directory for storing temporary data of the running VM.
      if [ -z "$TMPDIR" ] || [ -z "$USE_TMPDIR" ]; then
          TMPDIR=$(mktemp -d nix-vm.XXXXXXXXXX --tmpdir)
      fi

      ${lib.optionalString (cfg.useNixStoreImage)
        (if cfg.writableStore
          then ''
            # Create a writable copy/snapshot of the store image.
            ${qemu}/bin/qemu-img create -f qcow2 -F qcow2 -b ${storeImage}/nixos.qcow2 "$TMPDIR"/store.img
          ''
          else ''
            (
              cd ${builtins.storeDir}
              ${pkgs.erofs-utils}/bin/mkfs.erofs \
                --force-uid=0 \
                --force-gid=0 \
                -U eb176051-bd15-49b7-9e6b-462e0b467019 \
                -T 0 \
                --exclude-regex="$(
                  <${pkgs.closureInfo { rootPaths = [ config.system.build.toplevel regInfo ]; }}/store-paths \
                    sed -e 's^.*/^^g' \
                  | cut -c -10 \
                  | ${pkgs.python3}/bin/python ${./includes-to-excludes.py} )" \
                "$TMPDIR"/store.img \
                . \
                </dev/null >/dev/null
            )
          ''
        )
      }

      # Create a directory for exchanging data with the VM.
      mkdir -p "$TMPDIR/xchg"

      ${lib.optionalString cfg.useEFIBoot
      ''
        # Expose EFI variables, it's useful even when we are not using a bootloader (!).
        # We might be interested in having EFI variable storage present even if we aren't booting via UEFI, hence
        # no guard against `useBootLoader`.  Examples:
        # - testing PXE boot or other EFI applications
        # - directbooting LinuxBoot, which `kexec()s` into a UEFI environment that can boot e.g. Windows
        NIX_EFI_VARS=$(readlink -f "''${NIX_EFI_VARS:-${config.system.name}-efi-vars.fd}")
        # VM needs writable EFI vars
        if ! test -e "$NIX_EFI_VARS"; then
        ${if cfg.useBootLoader then
            # We still need the EFI var from the make-disk-image derivation
            # because our "switch-to-configuration" process might
            # write into it and we want to keep this data.
            ''cp ${systemImage}/efi-vars.fd "$NIX_EFI_VARS"''
            else
            ''cp ${cfg.efi.variables} "$NIX_EFI_VARS"''
          }
          chmod 0644 "$NIX_EFI_VARS"
        fi
      ''}

      cd "$TMPDIR"

      ${lib.optionalString (cfg.emptyDiskImages != []) "idx=0"}
      ${flip concatMapStrings cfg.emptyDiskImages (size: ''
        if ! test -e "empty$idx.qcow2"; then
            ${qemu}/bin/qemu-img create -f qcow2 "empty$idx.qcow2" "${toString size}M"
        fi
        idx=$((idx + 1))
      '')}

      # Start QEMU.
      exec ${qemu-common.qemuBinary qemu} \
          -name ${config.system.name} \
          -m ${toString config.virtualisation.memorySize} \
          -smp ${toString config.virtualisation.cores} \
          -device virtio-rng-pci \
          ${concatStringsSep " " config.virtualisation.qemu.networkingOptions} \
          ${concatStringsSep " \\\n    "
            (mapAttrsToList
              (tag: share: "-virtfs local,path=${share.source},security_model=none,mount_tag=${tag}")
              config.virtualisation.sharedDirectories)} \
          ${drivesCmdLine config.virtualisation.qemu.drives} \
          ${concatStringsSep " \\\n    " config.virtualisation.qemu.options} \
          $QEMU_OPTS \
          "$@"
    '';


  regInfo = pkgs.closureInfo { rootPaths = config.virtualisation.additionalPaths; };

  # System image is akin to a complete NixOS install with
  # a boot partition and root partition.
  systemImage = import ../../lib/make-disk-image.nix {
    inherit pkgs config lib;
    additionalPaths = [ regInfo ];
    format = "qcow2";
    onlyNixStore = false;
    partitionTableType = selectPartitionTableLayout { inherit (cfg) useDefaultFilesystems useEFIBoot; };
    # Bootloader should be installed on the system image only if we are booting through bootloaders.
    # Though, if a user is not using our default filesystems, it is possible to not have any ESP
    # or a strange partition table that's incompatible with GRUB configuration.
    # As a consequence, this may lead to disk image creation failures.
    # To avoid this, we prefer to let the user find out about how to install the bootloader on its ESP/disk.
    # Usually, this can be through building your own disk image.
    # TODO: If a user is interested into a more fine grained heuristic for `installBootLoader`
    # by examining the actual contents of `cfg.fileSystems`, please send a PR.
    installBootLoader = cfg.useBootLoader && cfg.useDefaultFilesystems;
    touchEFIVars = cfg.useEFIBoot;
    diskSize = "auto";
    additionalSpace = "0M";
    copyChannel = false;
    OVMF = cfg.efi.OVMF;
  };

  storeImage = import ../../lib/make-disk-image.nix {
    inherit pkgs config lib;
    additionalPaths = [ regInfo ];
    format = "qcow2";
    onlyNixStore = true;
    partitionTableType = "none";
    installBootLoader = false;
    touchEFIVars = false;
    diskSize = "auto";
    additionalSpace = "0M";
    copyChannel = false;
  };

  bootConfiguration =
    if cfg.useDefaultFilesystems
    then
      if cfg.useBootLoader
      then
        if cfg.useEFIBoot then "efi_bootloading_with_default_fs"
        else "legacy_bootloading_with_default_fs"
      else
        "direct_boot_with_default_fs"
    else
      "custom";
  suggestedRootDevice = {
    "efi_bootloading_with_default_fs" = "${cfg.bootLoaderDevice}2";
    "legacy_bootloading_with_default_fs" = "${cfg.bootLoaderDevice}1";
    "direct_boot_with_default_fs" = cfg.bootLoaderDevice;
    # This will enforce a NixOS module type checking error
    # to ask explicitly the user to set a rootDevice.
    # As it will look like `rootDevice = lib.mkDefault null;` after
    # all "computations".
    "custom" = null;
  }.${bootConfiguration};
in

{
  imports = [
    ../profiles/qemu-guest.nix
    (mkRenamedOptionModule [ "virtualisation" "pathsInNixDB" ] [ "virtualisation" "additionalPaths" ])
    (mkRemovedOptionModule [ "virtualisation" "bootDevice" ] "This option was renamed to `virtualisation.rootDevice`, as it was incorrectly named and misleading. Take the time to review what you want to do and look at the new options like `virtualisation.{bootLoaderDevice, bootPartition}`, open an issue in case of issues.")
    (mkRemovedOptionModule [ "virtualisation" "efiVars" ] "This option was removed, it is possible to provide a template UEFI variable with `virtualisation.efi.variables` ; if this option is important to you, open an issue")
    (mkRemovedOptionModule [ "virtualisation" "persistBootDevice" ] "Boot device is always persisted if you use a bootloader through the root disk image ; if this does not work for your usecase, please examine carefully what `virtualisation.{bootDevice, rootDevice, bootPartition}` options offer you and open an issue explaining your need.`")
  ];

  options = {

    virtualisation.fileSystems = options.fileSystems;

    virtualisation.memorySize =
      mkOption {
        type = types.ints.positive;
        default = 1024;
        description =
          lib.mdDoc ''
            The memory size in megabytes of the virtual machine.
          '';
      };

    virtualisation.msize =
      mkOption {
        type = types.ints.positive;
        default = 16384;
        description =
          lib.mdDoc ''
            The msize (maximum packet size) option passed to 9p file systems, in
            bytes. Increasing this should increase performance significantly,
            at the cost of higher RAM usage.
          '';
      };

    virtualisation.diskSize =
      mkOption {
        type = types.nullOr types.ints.positive;
        default = 1024;
        description =
          lib.mdDoc ''
            The disk size in megabytes of the virtual machine.
          '';
      };

    virtualisation.diskImage =
      mkOption {
        type = types.nullOr types.str;
        default = "./${config.system.name}.qcow2";
        defaultText = literalExpression ''"./''${config.system.name}.qcow2"'';
        description =
          lib.mdDoc ''
            Path to the disk image containing the root filesystem.
            The image will be created on startup if it does not
            exist.

            If null, a tmpfs will be used as the root filesystem and
            the VM's state will not be persistent.
          '';
      };

    virtualisation.bootLoaderDevice =
      mkOption {
        type = types.path;
        default = lookupDriveDeviceName "root" cfg.qemu.drives;
        defaultText = literalExpression ''lookupDriveDeviceName "root" cfg.qemu.drives'';
        example = "/dev/vda";
        description =
          lib.mdDoc ''
            The disk to be used for the boot filesystem.
            By default, it is the same disk as the root filesystem.
          '';
        };

    virtualisation.bootPartition =
      mkOption {
        type = types.nullOr types.path;
        default = if cfg.useEFIBoot then "${cfg.bootLoaderDevice}1" else null;
        defaultText = literalExpression ''if cfg.useEFIBoot then "''${cfg.bootLoaderDevice}1" else null'';
        example = "/dev/vda1";
        description =
          lib.mdDoc ''
            The boot partition to be used to mount /boot filesystem.
            In legacy boots, this should be null.
            By default, in EFI boot, it is the first partition of the boot device.
          '';
      };

    virtualisation.rootDevice =
      mkOption {
        type = types.nullOr types.path;
        example = "/dev/vda2";
        description =
          lib.mdDoc ''
            The disk or partition to be used for the root filesystem.
            By default (read the source code for more details):

            - under EFI with a bootloader: 2nd partition of the boot disk
            - in legacy boot with a bootloader: 1st partition of the boot disk
            - in direct boot (i.e. without a bootloader): whole disk

            In case you are not using a default boot device or a default filesystem, you have to set explicitly your root device.
          '';
      };

    virtualisation.emptyDiskImages =
      mkOption {
        type = types.listOf types.ints.positive;
        default = [];
        description =
          lib.mdDoc ''
            Additional disk images to provide to the VM. The value is
            a list of size in megabytes of each disk. These disks are
            writeable by the VM.
          '';
      };

    virtualisation.graphics =
      mkOption {
        type = types.bool;
        default = true;
        description =
          lib.mdDoc ''
            Whether to run QEMU with a graphics window, or in nographic mode.
            Serial console will be enabled on both settings, but this will
            change the preferred console.
            '';
      };

    virtualisation.resolution =
      mkOption {
        type = options.services.xserver.resolutions.type.nestedTypes.elemType;
        default = { x = 1024; y = 768; };
        description =
          lib.mdDoc ''
            The resolution of the virtual machine display.
          '';
      };

    virtualisation.cores =
      mkOption {
        type = types.ints.positive;
        default = 1;
        description =
          lib.mdDoc ''
            Specify the number of cores the guest is permitted to use.
            The number can be higher than the available cores on the
            host system.
          '';
      };

    virtualisation.sharedDirectories =
      mkOption {
        type = types.attrsOf
          (types.submodule {
            options.source = mkOption {
              type = types.str;
              description = lib.mdDoc "The path of the directory to share, can be a shell variable";
            };
            options.target = mkOption {
              type = types.path;
              description = lib.mdDoc "The mount point of the directory inside the virtual machine";
            };
          });
        default = { };
        example = {
          my-share = { source = "/path/to/be/shared"; target = "/mnt/shared"; };
        };
        description =
          lib.mdDoc ''
            An attributes set of directories that will be shared with the
            virtual machine using VirtFS (9P filesystem over VirtIO).
            The attribute name will be used as the 9P mount tag.
          '';
      };

    virtualisation.additionalPaths =
      mkOption {
        type = types.listOf types.path;
        default = [];
        description =
          lib.mdDoc ''
            A list of paths whose closure should be made available to
            the VM.

            When 9p is used, the closure is registered in the Nix
            database in the VM. All other paths in the host Nix store
            appear in the guest Nix store as well, but are considered
            garbage (because they are not registered in the Nix
            database of the guest).

            When {option}`virtualisation.useNixStoreImage` is
            set, the closure is copied to the Nix store image.
          '';
      };

    virtualisation.forwardPorts = mkOption {
      type = types.listOf
        (types.submodule {
          options.from = mkOption {
            type = types.enum [ "host" "guest" ];
            default = "host";
            description =
              lib.mdDoc ''
                Controls the direction in which the ports are mapped:

                - `"host"` means traffic from the host ports
                  is forwarded to the given guest port.
                - `"guest"` means traffic from the guest ports
                  is forwarded to the given host port.
              '';
          };
          options.proto = mkOption {
            type = types.enum [ "tcp" "udp" ];
            default = "tcp";
            description = lib.mdDoc "The protocol to forward.";
          };
          options.host.address = mkOption {
            type = types.str;
            default = "";
            description = lib.mdDoc "The IPv4 address of the host.";
          };
          options.host.port = mkOption {
            type = types.port;
            description = lib.mdDoc "The host port to be mapped.";
          };
          options.guest.address = mkOption {
            type = types.str;
            default = "";
            description = lib.mdDoc "The IPv4 address on the guest VLAN.";
          };
          options.guest.port = mkOption {
            type = types.port;
            description = lib.mdDoc "The guest port to be mapped.";
          };
        });
      default = [];
      example = lib.literalExpression
        ''
        [ # forward local port 2222 -> 22, to ssh into the VM
          { from = "host"; host.port = 2222; guest.port = 22; }

          # forward local port 80 -> 10.0.2.10:80 in the VLAN
          { from = "guest";
            guest.address = "10.0.2.10"; guest.port = 80;
            host.address = "127.0.0.1"; host.port = 80;
          }
        ]
        '';
      description =
        lib.mdDoc ''
          When using the SLiRP user networking (default), this option allows to
          forward ports to/from the host/guest.

          ::: {.warning}
          If the NixOS firewall on the virtual machine is enabled, you also
          have to open the guest ports to enable the traffic between host and
          guest.
          :::

          ::: {.note}
          Currently QEMU supports only IPv4 forwarding.
          :::
        '';
    };

    virtualisation.restrictNetwork =
      mkOption {
        type = types.bool;
        default = false;
        example = true;
        description =
          lib.mdDoc ''
            If this option is enabled, the guest will be isolated, i.e. it will
            not be able to contact the host and no guest IP packets will be
            routed over the host to the outside. This option does not affect
            any explicitly set forwarding rules.
          '';
      };

    virtualisation.vlans =
      mkOption {
        type = types.listOf types.ints.unsigned;
        default = [ 1 ];
        example = [ 1 2 ];
        description =
          lib.mdDoc ''
            Virtual networks to which the VM is connected.  Each
            number «N» in this list causes
            the VM to have a virtual Ethernet interface attached to a
            separate virtual network on which it will be assigned IP
            address
            `192.168.«N».«M»`,
            where «M» is the index of this VM
            in the list of VMs.
          '';
      };

    virtualisation.writableStore =
      mkOption {
        type = types.bool;
        default = cfg.mountHostNixStore;
        defaultText = literalExpression "cfg.mountHostNixStore";
        description =
          lib.mdDoc ''
            If enabled, the Nix store in the VM is made writable by
            layering an overlay filesystem on top of the host's Nix
            store.

            By default, this is enabled if you mount a host Nix store.
          '';
      };

    virtualisation.writableStoreUseTmpfs =
      mkOption {
        type = types.bool;
        default = true;
        description =
          lib.mdDoc ''
            Use a tmpfs for the writable store instead of writing to the VM's
            own filesystem.
          '';
      };

    networking.primaryIPAddress =
      mkOption {
        type = types.str;
        default = "";
        internal = true;
        description = lib.mdDoc "Primary IP address used in /etc/hosts.";
      };

    virtualisation.host.pkgs = mkOption {
      type = options.nixpkgs.pkgs.type;
      default = pkgs;
      defaultText = literalExpression "pkgs";
      example = literalExpression ''
        import pkgs.path { system = "x86_64-darwin"; }
      '';
      description = lib.mdDoc ''
        pkgs set to use for the host-specific packages of the vm runner.
        Changing this to e.g. a Darwin package set allows running NixOS VMs on Darwin.
      '';
    };

    virtualisation.qemu = {
      package =
        mkOption {
          type = types.package;
          default = cfg.host.pkgs.qemu_kvm;
          defaultText = literalExpression "config.virtualisation.host.pkgs.qemu_kvm";
          example = literalExpression "pkgs.qemu_test";
          description = lib.mdDoc "QEMU package to use.";
        };

      options =
        mkOption {
          type = types.listOf types.str;
          default = [];
          example = [ "-vga std" ];
          description = lib.mdDoc "Options passed to QEMU.";
        };

      consoles = mkOption {
        type = types.listOf types.str;
        default = let
          consoles = [ "${qemu-common.qemuSerialDevice},115200n8" "tty0" ];
        in if cfg.graphics then consoles else reverseList consoles;
        example = [ "console=tty1" ];
        description = lib.mdDoc ''
          The output console devices to pass to the kernel command line via the
          `console` parameter, the primary console is the last
          item of this list.

          By default it enables both serial console and
          `tty0`. The preferred console (last one) is based on
          the value of {option}`virtualisation.graphics`.
        '';
      };

      networkingOptions =
        mkOption {
          type = types.listOf types.str;
          default = [ ];
          example = [
            "-net nic,netdev=user.0,model=virtio"
            "-netdev user,id=user.0,\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
          ];
          description = lib.mdDoc ''
            Networking-related command-line options that should be passed to qemu.
            The default is to use userspace networking (SLiRP).

            If you override this option, be advised to keep
            ''${QEMU_NET_OPTS:+,$QEMU_NET_OPTS} (as seen in the example)
            to keep the default runtime behaviour.
          '';
        };

      drives =
        mkOption {
          type = types.listOf (types.submodule driveOpts);
          description = lib.mdDoc "Drives passed to qemu.";
          apply = addDeviceNames;
        };

      diskInterface =
        mkOption {
          type = types.enum [ "virtio" "scsi" "ide" ];
          default = "virtio";
          example = "scsi";
          description = lib.mdDoc "The interface used for the virtual hard disks.";
        };

      guestAgent.enable =
        mkOption {
          type = types.bool;
          default = true;
          description = lib.mdDoc ''
            Enable the Qemu guest agent.
          '';
        };

      virtioKeyboard =
        mkOption {
          type = types.bool;
          default = true;
          description = lib.mdDoc ''
            Enable the virtio-keyboard device.
          '';
        };
    };

    virtualisation.useNixStoreImage =
      mkOption {
        type = types.bool;
        default = false;
        description = lib.mdDoc ''
          Build and use a disk image for the Nix store, instead of
          accessing the host's one through 9p.

          For applications which do a lot of reads from the store,
          this can drastically improve performance, but at the cost of
          disk space and image build time.

          As an alternative, you can use a bootloader which will provide you
          with a full NixOS system image containing a Nix store and
          avoid mounting the host nix store through
          {option}`virtualisation.mountHostNixStore`.
        '';
      };

    virtualisation.mountHostNixStore =
      mkOption {
        type = types.bool;
        default = !cfg.useNixStoreImage && !cfg.useBootLoader;
        defaultText = literalExpression "!cfg.useNixStoreImage && !cfg.useBootLoader";
        description = lib.mdDoc ''
          Mount the host Nix store as a 9p mount.
        '';
      };

    virtualisation.useBootLoader =
      mkOption {
        type = types.bool;
        default = false;
        description =
          lib.mdDoc ''
            If enabled, the virtual machine will be booted using the
            regular boot loader (i.e., GRUB 1 or 2).  This allows
            testing of the boot loader.  If
            disabled (the default), the VM directly boots the NixOS
            kernel and initial ramdisk, bypassing the boot loader
            altogether.
          '';
      };

    virtualisation.useEFIBoot =
      mkOption {
        type = types.bool;
        default = false;
        description =
          lib.mdDoc ''
            If enabled, the virtual machine will provide a EFI boot
            manager.
            useEFIBoot is ignored if useBootLoader == false.
          '';
        };

    virtualisation.efi = {
      OVMF = mkOption {
        type = types.package;
        default = (pkgs.OVMF.override {
          secureBoot = cfg.useSecureBoot;
        }).fd;
        defaultText = ''(pkgs.OVMF.override {
          secureBoot = cfg.useSecureBoot;
        }).fd'';
        description =
        lib.mdDoc "OVMF firmware package, defaults to OVMF configured with secure boot if needed.";
      };

      firmware = mkOption {
        type = types.path;
        default = cfg.efi.OVMF.firmware;
        defaultText = literalExpression "cfg.efi.OVMF.firmware";
        description =
          lib.mdDoc ''
            Firmware binary for EFI implementation, defaults to OVMF.
          '';
      };

      variables = mkOption {
        type = types.path;
        default = cfg.efi.OVMF.variables;
        defaultText = literalExpression "cfg.efi.OVMF.variables";
        description =
          lib.mdDoc ''
            Platform-specific flash binary for EFI variables, implementation-dependent to the EFI firmware.
            Defaults to OVMF.
          '';
      };
    };

    virtualisation.useDefaultFilesystems =
      mkOption {
        type = types.bool;
        default = true;
        description =
          lib.mdDoc ''
            If enabled, the boot disk of the virtual machine will be
            formatted and mounted with the default filesystems for
            testing. Swap devices and LUKS will be disabled.

            If disabled, a root filesystem has to be specified and
            formatted (for example in the initial ramdisk).
          '';
      };

    virtualisation.useSecureBoot =
      mkOption {
        type = types.bool;
        default = false;
        description =
          lib.mdDoc ''
            Enable Secure Boot support in the EFI firmware.
          '';
      };


    virtualisation.bios =
      mkOption {
        type = types.nullOr types.package;
        default = null;
        description =
          lib.mdDoc ''
            An alternate BIOS (such as `qboot`) with which to start the VM.
            Should contain a file named `bios.bin`.
            If `null`, QEMU's builtin SeaBIOS will be used.
          '';
      };

  };

  config = {

    assertions =
      lib.concatLists (lib.flip lib.imap cfg.forwardPorts (i: rule:
        [
          { assertion = rule.from == "guest" -> rule.proto == "tcp";
            message =
              ''
                Invalid virtualisation.forwardPorts.<entry ${toString i}>.proto:
                  Guest forwarding supports only TCP connections.
              '';
          }
          { assertion = rule.from == "guest" -> lib.hasPrefix "10.0.2." rule.guest.address;
            message =
              ''
                Invalid virtualisation.forwardPorts.<entry ${toString i}>.guest.address:
                  The address must be in the default VLAN (10.0.2.0/24).
              '';
          }
        ])) ++ [
          { assertion = pkgs.stdenv.hostPlatform.is32bit -> cfg.memorySize < 2047;
            message = ''
              virtualisation.memorySize is above 2047, but qemu is only able to allocate 2047MB RAM on 32bit max.
            '';
          }
        ];

    warnings =
      optional (
        cfg.writableStore &&
        cfg.useNixStoreImage &&
        opt.writableStore.highestPrio > lib.modules.defaultOverridePriority)
        ''
          You have enabled ${opt.useNixStoreImage} = true,
          without setting ${opt.writableStore} = false.

          This causes a store image to be written to the store, which is
          costly, especially for the binary cache, and because of the need
          for more frequent garbage collection.

          If you really need this combination, you can set ${opt.writableStore}
          explicitly to true, incur the cost and make this warning go away.
          Otherwise, we recommend

            ${opt.writableStore} = false;
        '';

    # In UEFI boot, we use a EFI-only partition table layout, thus GRUB will fail when trying to install
    # legacy and UEFI. In order to avoid this, we have to put "nodev" to force UEFI-only installs.
    # Otherwise, we set the proper bootloader device for this.
    # FIXME: make a sense of this mess wrt to multiple ESP present in the system, probably use boot.efiSysMountpoint?
    boot.loader.grub.device = mkVMOverride (if cfg.useEFIBoot then "nodev" else cfg.bootLoaderDevice);
    boot.loader.grub.gfxmodeBios = with cfg.resolution; "${toString x}x${toString y}";
    virtualisation.rootDevice = mkDefault suggestedRootDevice;

    boot.initrd.kernelModules = optionals (cfg.useNixStoreImage && !cfg.writableStore) [ "erofs" ];

    boot.loader.supportsInitrdSecrets = mkIf (!cfg.useBootLoader) (mkVMOverride false);

    boot.initrd.extraUtilsCommands = lib.mkIf (cfg.useDefaultFilesystems && !config.boot.initrd.systemd.enable)
      ''
        # We need mke2fs in the initrd.
        copy_bin_and_libs ${pkgs.e2fsprogs}/bin/mke2fs
      '';

    boot.initrd.postDeviceCommands = lib.mkIf (cfg.useDefaultFilesystems && !config.boot.initrd.systemd.enable)
      ''
        # If the disk image appears to be empty, run mke2fs to
        # initialise.
        FSTYPE=$(blkid -o value -s TYPE ${cfg.rootDevice} || true)
        PARTTYPE=$(blkid -o value -s PTTYPE ${cfg.rootDevice} || true)
        if test -z "$FSTYPE" -a -z "$PARTTYPE"; then
            mke2fs -t ext4 ${cfg.rootDevice}
        fi
      '';

    boot.initrd.postMountCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
      ''
        # Mark this as a NixOS machine.
        mkdir -p $targetRoot/etc
        echo -n > $targetRoot/etc/NIXOS

        # Fix the permissions on /tmp.
        chmod 1777 $targetRoot/tmp

        mkdir -p $targetRoot/boot

        ${optionalString cfg.writableStore ''
          echo "mounting overlay filesystem on /nix/store..."
          mkdir -p -m 0755 $targetRoot/nix/.rw-store/store $targetRoot/nix/.rw-store/work $targetRoot/nix/store
          mount -t overlay overlay $targetRoot/nix/store \
            -o lowerdir=$targetRoot/nix/.ro-store,upperdir=$targetRoot/nix/.rw-store/store,workdir=$targetRoot/nix/.rw-store/work || fail
        ''}
      '';

    systemd.tmpfiles.rules = lib.mkIf config.boot.initrd.systemd.enable [
      "f /etc/NIXOS 0644 root root -"
      "d /boot 0644 root root -"
    ];

    # After booting, register the closure of the paths in
    # `virtualisation.additionalPaths' in the Nix database in the VM.  This
    # allows Nix operations to work in the VM.  The path to the
    # registration file is passed through the kernel command line to
    # allow `system.build.toplevel' to be included.  (If we had a direct
    # reference to ${regInfo} here, then we would get a cyclic
    # dependency.)
    boot.postBootCommands = lib.mkIf config.nix.enable
      ''
        if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then
          ${config.nix.package.out}/bin/nix-store --load-db < ''${BASH_REMATCH[1]}
        fi
      '';

    boot.initrd.availableKernelModules =
      optional cfg.writableStore "overlay"
      ++ optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx";

    virtualisation.additionalPaths = [ config.system.build.toplevel ];

    virtualisation.sharedDirectories = {
      nix-store = mkIf cfg.mountHostNixStore {
        source = builtins.storeDir;
        target = "/nix/store";
      };
      xchg = {
        source = ''"$TMPDIR"/xchg'';
        target = "/tmp/xchg";
      };
      shared = {
        source = ''"''${SHARED_DIR:-$TMPDIR/xchg}"'';
        target = "/tmp/shared";
      };
    };

    virtualisation.qemu.networkingOptions =
      let
        forwardingOptions = flip concatMapStrings cfg.forwardPorts
          ({ proto, from, host, guest }:
            if from == "host"
              then "hostfwd=${proto}:${host.address}:${toString host.port}-" +
                   "${guest.address}:${toString guest.port},"
              else "'guestfwd=${proto}:${guest.address}:${toString guest.port}-" +
                   "cmd:${pkgs.netcat}/bin/nc ${host.address} ${toString host.port}',"
          );
        restrictNetworkOption = lib.optionalString cfg.restrictNetwork "restrict=on,";
      in
      [
        "-net nic,netdev=user.0,model=virtio"
        "-netdev user,id=user.0,${forwardingOptions}${restrictNetworkOption}\"$QEMU_NET_OPTS\""
      ];

    # FIXME: Consolidate this one day.
    virtualisation.qemu.options = mkMerge [
      (mkIf cfg.qemu.virtioKeyboard [
        "-device virtio-keyboard"
      ])
      (mkIf pkgs.stdenv.hostPlatform.isx86 [
        "-usb" "-device usb-tablet,bus=usb-bus.0"
      ])
      (mkIf pkgs.stdenv.hostPlatform.isAarch [
        "-device virtio-gpu-pci" "-device usb-ehci,id=usb0" "-device usb-kbd" "-device usb-tablet"
      ])
      (let
        alphaNumericChars = lowerChars ++ upperChars ++ (map toString (range 0 9));
        # Replace all non-alphanumeric characters with underscores
        sanitizeShellIdent = s: concatMapStrings (c: if builtins.elem c alphaNumericChars then c else "_") (stringToCharacters s);
      in mkIf (!cfg.useBootLoader) [
        "-kernel \${NIXPKGS_QEMU_KERNEL_${sanitizeShellIdent config.system.name}:-${config.system.build.toplevel}/kernel}"
        "-initrd ${config.system.build.toplevel}/initrd"
        ''-append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.toplevel}/init regInfo=${regInfo}/registration ${consoles} $QEMU_KERNEL_PARAMS"''
      ])
      (mkIf cfg.useEFIBoot [
        "-drive if=pflash,format=raw,unit=0,readonly=on,file=${cfg.efi.firmware}"
        "-drive if=pflash,format=raw,unit=1,readonly=off,file=$NIX_EFI_VARS"
      ])
      (mkIf (cfg.bios != null) [
        "-bios ${cfg.bios}/bios.bin"
      ])
      (mkIf (!cfg.graphics) [
        "-nographic"
      ])
    ];

    virtualisation.qemu.drives = mkMerge [
      (mkIf (cfg.diskImage != null) [{
        name = "root";
        file = ''"$NIX_DISK_IMAGE"'';
        driveExtraOpts.cache = "writeback";
        driveExtraOpts.werror = "report";
        deviceExtraOpts.bootindex = "1";
      }])
      (mkIf cfg.useNixStoreImage [{
        name = "nix-store";
        file = ''"$TMPDIR"/store.img'';
        deviceExtraOpts.bootindex = "2";
        driveExtraOpts.format = if cfg.writableStore then "qcow2" else "raw";
      }])
      (imap0 (idx: _: {
        file = "$(pwd)/empty${toString idx}.qcow2";
        driveExtraOpts.werror = "report";
      }) cfg.emptyDiskImages)
    ];

    fileSystems = mkVMOverride cfg.fileSystems;

    # Mount the host filesystem via 9P, and bind-mount the Nix store
    # of the host into our own filesystem.  We use mkVMOverride to
    # allow this module to be applied to "normal" NixOS system
    # configuration, where the regular value for the `fileSystems'
    # attribute should be disregarded for the purpose of building a VM
    # test image (since those filesystems don't exist in the VM).
    virtualisation.fileSystems = let
      mkSharedDir = tag: share:
        {
          name =
            if tag == "nix-store" && cfg.writableStore
            then "/nix/.ro-store"
            else share.target;
          value.device = tag;
          value.fsType = "9p";
          value.neededForBoot = true;
          value.options =
            [ "trans=virtio" "version=9p2000.L"  "msize=${toString cfg.msize}" ]
            ++ lib.optional (tag == "nix-store") "cache=loose";
        };
    in lib.mkMerge [
      (lib.mapAttrs' mkSharedDir cfg.sharedDirectories)
      {
        "/" = lib.mkIf cfg.useDefaultFilesystems (if cfg.diskImage == null then {
          device = "tmpfs";
          fsType = "tmpfs";
        } else {
          device = cfg.rootDevice;
          fsType = "ext4";
          autoFormat = true;
        });
        "/tmp" = lib.mkIf config.boot.tmp.useTmpfs {
          device = "tmpfs";
          fsType = "tmpfs";
          neededForBoot = true;
          # Sync with systemd's tmp.mount;
          options = [ "mode=1777" "strictatime" "nosuid" "nodev" "size=${toString config.boot.tmp.tmpfsSize}" ];
        };
        "/nix/${if cfg.writableStore then ".ro-store" else "store"}" = lib.mkIf cfg.useNixStoreImage {
          device = "${lookupDriveDeviceName "nix-store" cfg.qemu.drives}";
          neededForBoot = true;
          options = [ "ro" ];
        };
        "/nix/.rw-store" = lib.mkIf (cfg.writableStore && cfg.writableStoreUseTmpfs) {
          fsType = "tmpfs";
          options = [ "mode=0755" ];
          neededForBoot = true;
        };
        "/boot" = lib.mkIf (cfg.useBootLoader && cfg.bootPartition != null) {
          device = cfg.bootPartition; # 1 for e.g. `vda1`, as created in `systemImage`
          fsType = "vfat";
          noCheck = true; # fsck fails on a r/o filesystem
        };
      }
    ];

    boot.initrd.systemd = lib.mkIf (config.boot.initrd.systemd.enable && cfg.writableStore) {
      mounts = [{
        where = "/sysroot/nix/store";
        what = "overlay";
        type = "overlay";
        options = "lowerdir=/sysroot/nix/.ro-store,upperdir=/sysroot/nix/.rw-store/store,workdir=/sysroot/nix/.rw-store/work";
        wantedBy = ["initrd-fs.target"];
        before = ["initrd-fs.target"];
        requires = ["rw-store.service"];
        after = ["rw-store.service"];
        unitConfig.RequiresMountsFor = "/sysroot/nix/.ro-store";
      }];
      services.rw-store = {
        unitConfig = {
          DefaultDependencies = false;
          RequiresMountsFor = "/sysroot/nix/.rw-store";
        };
        serviceConfig = {
          Type = "oneshot";
          ExecStart = "/bin/mkdir -p -m 0755 /sysroot/nix/.rw-store/store /sysroot/nix/.rw-store/work /sysroot/nix/store";
        };
      };
    };

    swapDevices = (if cfg.useDefaultFilesystems then mkVMOverride else mkDefault) [ ];
    boot.initrd.luks.devices = (if cfg.useDefaultFilesystems then mkVMOverride else mkDefault) {};

    # Don't run ntpd in the guest.  It should get the correct time from KVM.
    services.timesyncd.enable = false;

    services.qemuGuest.enable = cfg.qemu.guestAgent.enable;

    system.build.vm = cfg.host.pkgs.runCommand "nixos-vm" {
      preferLocalBuild = true;
      meta.mainProgram = "run-${config.system.name}-vm";
    }
      ''
        mkdir -p $out/bin
        ln -s ${config.system.build.toplevel} $out/system
        ln -s ${cfg.host.pkgs.writeScript "run-nixos-vm" startVM} $out/bin/run-${config.system.name}-vm
      '';

    # When building a regular system configuration, override whatever
    # video driver the host uses.
    services.xserver.videoDrivers = mkVMOverride [ "modesetting" ];
    services.xserver.defaultDepth = mkVMOverride 0;
    services.xserver.resolutions = mkVMOverride [ cfg.resolution ];
    services.xserver.monitorSection =
      ''
        # Set a higher refresh rate so that resolutions > 800x600 work.
        HorizSync 30-140
        VertRefresh 50-160
      '';

    # Wireless won't work in the VM.
    networking.wireless.enable = mkVMOverride false;
    services.connman.enable = mkVMOverride false;

    # Speed up booting by not waiting for ARP.
    networking.dhcpcd.extraConfig = "noarp";

    networking.usePredictableInterfaceNames = false;

    system.requiredKernelConfig = with config.lib.kernelConfig;
      [ (isEnabled "VIRTIO_BLK")
        (isEnabled "VIRTIO_PCI")
        (isEnabled "VIRTIO_NET")
        (isEnabled "EXT4_FS")
        (isEnabled "NET_9P_VIRTIO")
        (isEnabled "9P_FS")
        (isYes "BLK_DEV")
        (isYes "PCI")
        (isYes "NETDEVICES")
        (isYes "NET_CORE")
        (isYes "INET")
        (isYes "NETWORK_FILESYSTEMS")
      ] ++ optionals (!cfg.graphics) [
        (isYes "SERIAL_8250_CONSOLE")
        (isYes "SERIAL_8250")
      ] ++ optionals (cfg.writableStore) [
        (isEnabled "OVERLAY_FS")
      ];

  };

  # uses types of services/x11/xserver.nix
  meta.buildDocsInSandbox = false;
}