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#

# Copyright (C) 2006, 2007, 2008, 2009, 2010 Google Inc.

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# General Public License for more details.

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"""Xen hypervisors

"""

import logging

from cStringIO import StringIO

from ganeti import constants

from ganeti import errors

from ganeti import utils

from ganeti.hypervisor import hv_base

from ganeti import netutils

from ganeti import objects

class XenHypervisor(hv_base.BaseHypervisor):

  """Xen generic hypervisor interface

  This is the Xen base class used for both Xen PVM and HVM. It contains

  all the functionality that is identical for both.

  """

  CAN_MIGRATE = True

  REBOOT_RETRY_COUNT = 60

  REBOOT_RETRY_INTERVAL = 10

  ANCILLARY_FILES = [

    "/etc/xen/xend-config.sxp",

    "/etc/xen/scripts/vif-bridge",

    ]

  @classmethod

  def _WriteConfigFile(cls, instance, block_devices):

    """Write the Xen config file for the instance.

    """

    raise NotImplementedError

  @staticmethod

  def _WriteConfigFileStatic(instance_name, data):

    """Write the Xen config file for the instance.

    This version of the function just writes the config file from static data.

    """

    utils.WriteFile("/etc/xen/%s" % instance_name, data=data)

  @staticmethod

  def _ReadConfigFile(instance_name):

    """Returns the contents of the instance config file.

    """

    try:

      file_content = utils.ReadFile("/etc/xen/%s" % instance_name)

    except EnvironmentError, err:

      raise errors.HypervisorError("Failed to load Xen config file: %s" % err)

    return file_content

  @staticmethod

  def _RemoveConfigFile(instance_name):

    """Remove the xen configuration file.

    """

    utils.RemoveFile("/etc/xen/%s" % instance_name)

  @classmethod

  def _CreateConfigCpus(cls, cpu_mask):

    """Create a CPU config string that's compatible with Xen's

    configuration file.

    """

    # Convert the string CPU mask to a list of list of int's

    cpu_list = utils.ParseMultiCpuMask(cpu_mask)

    if len(cpu_list) == 1:

      all_cpu_mapping = cpu_list[0]

      if all_cpu_mapping == constants.CPU_PINNING_OFF:

        # If CPU pinning has 1 entry that's "all", then remove the

        # parameter from the config file

        return None

      else:

        # If CPU pinning has one non-all entry, mapping all vCPUS (the entire

        # VM) to one physical CPU, using format 'cpu = "C"'

        return "cpu = \"%s\"" % ",".join(map(str, all_cpu_mapping))

    else:

      def _GetCPUMap(vcpu):

        if vcpu[0] == constants.CPU_PINNING_ALL_VAL:

          cpu_map = constants.CPU_PINNING_ALL_XEN

        else:

          cpu_map = ",".join(map(str, vcpu))

        return "\"%s\"" % cpu_map

      # build the result string in format 'cpus = [ "c", "c", "c" ]',

      # where each c is a physical CPU number, a range, a list, or any

      # combination

      return "cpus = [ %s ]" % ", ".join(map(_GetCPUMap, cpu_list))

  @staticmethod

  def _RunXmList(xmlist_errors):

    """Helper function for L{_GetXMList} to run "xm list".

    """

    result = utils.RunCmd(["xm", "list"])

    if result.failed:

      logging.error("xm list failed (%s): %s", result.fail_reason,

                    result.output)

      xmlist_errors.append(result)

      raise utils.RetryAgain()

    # skip over the heading

    return result.stdout.splitlines()[1:]

  @classmethod

  def _GetXMList(cls, include_node):

    """Return the list of running instances.

    If the include_node argument is True, then we return information

    for dom0 also, otherwise we filter that from the return value.

    @return: list of (name, id, memory, vcpus, state, time spent)

    """

    xmlist_errors = []

    try:

      lines = utils.Retry(cls._RunXmList, 1, 5, args=(xmlist_errors, ))

    except utils.RetryTimeout:

      if xmlist_errors:

        xmlist_result = xmlist_errors.pop()

        errmsg = ("xm list failed, timeout exceeded (%s): %s" %

                  (xmlist_result.fail_reason, xmlist_result.output))

      else:

        errmsg = "xm list failed"

      raise errors.HypervisorError(errmsg)

    result = []

    for line in lines:

      # The format of lines is:

      # Name      ID Mem(MiB) VCPUs State  Time(s)

      # Domain-0   0  3418     4 r-----    266.2

      data = line.split()

      if len(data) != 6:

        raise errors.HypervisorError("Can't parse output of xm list,"

                                     " line: %s" % line)

      try:

        data[1] = int(data[1])

        data[2] = int(data[2])

        data[3] = int(data[3])

        data[5] = float(data[5])

      except (TypeError, ValueError), err:

        raise errors.HypervisorError("Can't parse output of xm list,"

                                     " line: %s, error: %s" % (line, err))

      # skip the Domain-0 (optional)

      if include_node or data[0] != "Domain-0":

        result.append(data)

    return result

  def ListInstances(self):

    """Get the list of running instances.

    """

    xm_list = self._GetXMList(False)

    names = [info[0] for info in xm_list]

    return names

  def GetInstanceInfo(self, instance_name):

    """Get instance properties.

    @param instance_name: the instance name

    @return: tuple (name, id, memory, vcpus, stat, times)

    """

    xm_list = self._GetXMList(instance_name == "Domain-0")

    result = None

    for data in xm_list:

      if data[0] == instance_name:

        result = data

        break

    return result

  def GetAllInstancesInfo(self):

    """Get properties of all instances.

    @return: list of tuples (name, id, memory, vcpus, stat, times)

    """

    xm_list = self._GetXMList(False)

    return xm_list

  def StartInstance(self, instance, block_devices, startup_paused):

    """Start an instance.

    """

    self._WriteConfigFile(instance, block_devices)

    cmd = ["xm", "create"]

    if startup_paused:

      cmd.extend(["--paused"])

    cmd.extend([instance.name])

    result = utils.RunCmd(cmd)

    if result.failed:

      raise errors.HypervisorError("Failed to start instance %s: %s (%s)" %

                                   (instance.name, result.fail_reason,

                                    result.output))

  def StopInstance(self, instance, force=False, retry=False, name=None):

    """Stop an instance.

    """

    if name is None:

      name = instance.name

    self._RemoveConfigFile(name)

    if force:

      command = ["xm", "destroy", name]

    else:

      command = ["xm", "shutdown", name]

    result = utils.RunCmd(command)

    if result.failed:

      raise errors.HypervisorError("Failed to stop instance %s: %s, %s" %

                                   (name, result.fail_reason, result.output))

  def RebootInstance(self, instance):

    """Reboot an instance.

    """

    ini_info = self.GetInstanceInfo(instance.name)

    if ini_info is None:

      raise errors.HypervisorError("Failed to reboot instance %s,"

                                   " not running" % instance.name)

    result = utils.RunCmd(["xm", "reboot", instance.name])

    if result.failed:

      raise errors.HypervisorError("Failed to reboot instance %s: %s, %s" %

                                   (instance.name, result.fail_reason,

                                    result.output))

    def _CheckInstance():

      new_info = self.GetInstanceInfo(instance.name)

      # check if the domain ID has changed or the run time has decreased

      if (new_info is not None and

          (new_info[1] != ini_info[1] or new_info[5] < ini_info[5])):

        return

      raise utils.RetryAgain()

    try:

      utils.Retry(_CheckInstance, self.REBOOT_RETRY_INTERVAL,

                  self.REBOOT_RETRY_INTERVAL * self.REBOOT_RETRY_COUNT)

    except utils.RetryTimeout:

      raise errors.HypervisorError("Failed to reboot instance %s: instance"

                                   " did not reboot in the expected interval" %

                                   (instance.name, ))

  def GetNodeInfo(self):

    """Return information about the node.

    @return: a dict with the following keys (memory values in MiB):

          - memory_total: the total memory size on the node

          - memory_free: the available memory on the node for instances

          - memory_dom0: the memory used by the node itself, if available

          - nr_cpus: total number of CPUs

          - nr_nodes: in a NUMA system, the number of domains

          - nr_sockets: the number of physical CPU sockets in the node

    """

    # note: in xen 3, memory has changed to total_memory

    result = utils.RunCmd(["xm", "info"])

    if result.failed:

      logging.error("Can't run 'xm info' (%s): %s", result.fail_reason,

                    result.output)

      return None

    xmoutput = result.stdout.splitlines()

    result = {}

    cores_per_socket = threads_per_core = nr_cpus = None

    for line in xmoutput:

      splitfields = line.split(":", 1)

      if len(splitfields) > 1:

        key = splitfields[0].strip()

        val = splitfields[1].strip()

        if key == "memory" or key == "total_memory":

          result["memory_total"] = int(val)

        elif key == "free_memory":

          result["memory_free"] = int(val)

        elif key == "nr_cpus":

          nr_cpus = result["cpu_total"] = int(val)

        elif key == "nr_nodes":

          result["cpu_nodes"] = int(val)

        elif key == "cores_per_socket":

          cores_per_socket = int(val)

        elif key == "threads_per_core":

          threads_per_core = int(val)

    if (cores_per_socket is not None and

        threads_per_core is not None and nr_cpus is not None):

      result["cpu_sockets"] = nr_cpus / (cores_per_socket * threads_per_core)

    dom0_info = self.GetInstanceInfo("Domain-0")

    if dom0_info is not None:

      result["memory_dom0"] = dom0_info[2]

    return result

  @classmethod

  def GetInstanceConsole(cls, instance, hvparams, beparams):

    """Return a command for connecting to the console of an instance.

    """

    return objects.InstanceConsole(instance=instance.name,

                                   kind=constants.CONS_SSH,

                                   host=instance.primary_node,

                                   user=constants.GANETI_RUNAS,

                                   command=[constants.XM_CONSOLE_WRAPPER,

                                            instance.name])

  def Verify(self):

    """Verify the hypervisor.

    For Xen, this verifies that the xend process is running.

    """

    result = utils.RunCmd(["xm", "info"])

    if result.failed:

      return "'xm info' failed: %s, %s" % (result.fail_reason, result.output)

  @staticmethod

  def _GetConfigFileDiskData(block_devices, blockdev_prefix):

    """Get disk directive for xen config file.

    This method builds the xen config disk directive according to the

    given disk_template and block_devices.

    @param block_devices: list of tuples (cfdev, rldev):

        - cfdev: dict containing ganeti config disk part

        - rldev: ganeti.bdev.BlockDev object

    @param blockdev_prefix: a string containing blockdevice prefix,

                            e.g. "sd" for /dev/sda

    @return: string containing disk directive for xen instance config file

    """

    FILE_DRIVER_MAP = {

      constants.FD_LOOP: "file",

      constants.FD_BLKTAP: "tap:aio",

      }

    disk_data = []

    if len(block_devices) > 24:

      # 'z' - 'a' = 24

      raise errors.HypervisorError("Too many disks")

    namespace = [blockdev_prefix + chr(i + ord("a")) for i in range(24)]

    for sd_name, (cfdev, dev_path) in zip(namespace, block_devices):

      if cfdev.mode == constants.DISK_RDWR:

        mode = "w"

      else:

        mode = "r"

      if cfdev.dev_type == constants.LD_FILE:

        line = "'%s:%s,%s,%s'" % (FILE_DRIVER_MAP[cfdev.physical_id[0]],

                                  dev_path, sd_name, mode)

      else:

        line = "'phy:%s,%s,%s'" % (dev_path, sd_name, mode)

      disk_data.append(line)

    return disk_data

  def MigrationInfo(self, instance):

    """Get instance information to perform a migration.

    @type instance: L{objects.Instance}

    @param instance: instance to be migrated

    @rtype: string

    @return: content of the xen config file

    """

    return self._ReadConfigFile(instance.name)

  def AcceptInstance(self, instance, info, target):

    """Prepare to accept an instance.

    @type instance: L{objects.Instance}

    @param instance: instance to be accepted

    @type info: string

    @param info: content of the xen config file on the source node

    @type target: string

    @param target: target host (usually ip), on this node

    """

    pass

  def FinalizeMigration(self, instance, info, success):

    """Finalize an instance migration.

    After a successful migration we write the xen config file.

    We do nothing on a failure, as we did not change anything at accept time.

    @type instance: L{objects.Instance}

    @param instance: instance whose migration is being finalized

    @type info: string

    @param info: content of the xen config file on the source node

    @type success: boolean

    @param success: whether the migration was a success or a failure

    """

    if success:

      self._WriteConfigFileStatic(instance.name, info)

  def MigrateInstance(self, instance, target, live):

    """Migrate an instance to a target node.

    The migration will not be attempted if the instance is not

    currently running.

    @type instance: L{objects.Instance}

    @param instance: the instance to be migrated

    @type target: string

    @param target: ip address of the target node

    @type live: boolean

    @param live: perform a live migration

    """

    if self.GetInstanceInfo(instance.name) is None:

      raise errors.HypervisorError("Instance not running, cannot migrate")

    port = instance.hvparams[constants.HV_MIGRATION_PORT]

    if not netutils.TcpPing(target, port, live_port_needed=True):

      raise errors.HypervisorError("Remote host %s not listening on port"

                                   " %s, cannot migrate" % (target, port))

    args = ["xm", "migrate", "-p", "%d" % port]

    if live:

      args.append("-l")

    args.extend([instance.name, target])

    result = utils.RunCmd(args)

    if result.failed:

      raise errors.HypervisorError("Failed to migrate instance %s: %s" %

                                   (instance.name, result.output))

    # remove old xen file after migration succeeded

    try:

      self._RemoveConfigFile(instance.name)

    except EnvironmentError:

      logging.exception("Failure while removing instance config file")

  @classmethod

  def PowercycleNode(cls):

    """Xen-specific powercycle.

    This first does a Linux reboot (which triggers automatically a Xen

    reboot), and if that fails it tries to do a Xen reboot. The reason

    we don't try a Xen reboot first is that the xen reboot launches an

    external command which connects to the Xen hypervisor, and that

    won't work in case the root filesystem is broken and/or the xend

    daemon is not working.

    """

    try:

      cls.LinuxPowercycle()

    finally:

      utils.RunCmd(["xm", "debug", "R"])

class XenPvmHypervisor(XenHypervisor):

  """Xen PVM hypervisor interface"""

  PARAMETERS = {

    constants.HV_USE_BOOTLOADER: hv_base.NO_CHECK,

    constants.HV_BOOTLOADER_PATH: hv_base.OPT_FILE_CHECK,

    constants.HV_BOOTLOADER_ARGS: hv_base.NO_CHECK,

    constants.HV_KERNEL_PATH: hv_base.REQ_FILE_CHECK,

    constants.HV_INITRD_PATH: hv_base.OPT_FILE_CHECK,

    constants.HV_ROOT_PATH: hv_base.NO_CHECK,

    constants.HV_KERNEL_ARGS: hv_base.NO_CHECK,

    constants.HV_MIGRATION_PORT: hv_base.REQ_NET_PORT_CHECK,

    constants.HV_MIGRATION_MODE: hv_base.MIGRATION_MODE_CHECK,

    # TODO: Add a check for the blockdev prefix (matching [a-z:] or similar).

    constants.HV_BLOCKDEV_PREFIX: hv_base.NO_CHECK,

    constants.HV_REBOOT_BEHAVIOR:

      hv_base.ParamInSet(True, constants.REBOOT_BEHAVIORS),

    constants.HV_CPU_MASK: hv_base.OPT_MULTI_CPU_MASK_CHECK,

    }

  @classmethod

  def _WriteConfigFile(cls, instance, block_devices):

    """Write the Xen config file for the instance.

    """

    hvp = instance.hvparams

    config = StringIO()

    config.write("# this is autogenerated by Ganeti, please do not edit\n#\n")

    # if bootloader is True, use bootloader instead of kernel and ramdisk

    # parameters.

    if hvp[constants.HV_USE_BOOTLOADER]:

      # bootloader handling

      bootloader_path = hvp[constants.HV_BOOTLOADER_PATH]

      if bootloader_path:

        config.write("bootloader = '%s'\n" % bootloader_path)

      else:

        raise errors.HypervisorError("Bootloader enabled, but missing"

                                     " bootloader path")

      bootloader_args = hvp[constants.HV_BOOTLOADER_ARGS]

      if bootloader_args:

        config.write("bootargs = '%s'\n" % bootloader_args)

    else:

      # kernel handling

      kpath = hvp[constants.HV_KERNEL_PATH]

      config.write("kernel = '%s'\n" % kpath)

      # initrd handling

      initrd_path = hvp[constants.HV_INITRD_PATH]

      if initrd_path:

        config.write("ramdisk = '%s'\n" % initrd_path)

    # rest of the settings

    config.write("memory = %d\n" % instance.beparams[constants.BE_MEMORY])

    config.write("vcpus = %d\n" % instance.beparams[constants.BE_VCPUS])

    cpu_pinning = cls._CreateConfigCpus(hvp[constants.HV_CPU_MASK])

    if cpu_pinning:

      config.write("%s\n" % cpu_pinning)

    config.write("name = '%s'\n" % instance.name)

    vif_data = []

    for nic in instance.nics:

      nic_str = "mac=%s" % (nic.mac)

      ip = getattr(nic, "ip", None)

      if ip is not None:

        nic_str += ", ip=%s" % ip

      if nic.nicparams[constants.NIC_MODE] == constants.NIC_MODE_BRIDGED:

        nic_str += ", bridge=%s" % nic.nicparams[constants.NIC_LINK]

      vif_data.append("'%s'" % nic_str)

    disk_data = cls._GetConfigFileDiskData(block_devices,

                                           hvp[constants.HV_BLOCKDEV_PREFIX])

    config.write("vif = [%s]\n" % ",".join(vif_data))

    config.write("disk = [%s]\n" % ",".join(disk_data))

    if hvp[constants.HV_ROOT_PATH]:

      config.write("root = '%s'\n" % hvp[constants.HV_ROOT_PATH])

    config.write("on_poweroff = 'destroy'\n")

    if hvp[constants.HV_REBOOT_BEHAVIOR] == constants.INSTANCE_REBOOT_ALLOWED:

      config.write("on_reboot = 'restart'\n")

    else:

      config.write("on_reboot = 'destroy'\n")

    config.write("on_crash = 'restart'\n")

    config.write("extra = '%s'\n" % hvp[constants.HV_KERNEL_ARGS])

    # just in case it exists

    utils.RemoveFile("/etc/xen/auto/%s" % instance.name)

    try:

      utils.WriteFile("/etc/xen/%s" % instance.name, data=config.getvalue())

    except EnvironmentError, err:

      raise errors.HypervisorError("Cannot write Xen instance confile"

                                   " file /etc/xen/%s: %s" %

                                   (instance.name, err))

    return True

class XenHvmHypervisor(XenHypervisor):

  """Xen HVM hypervisor interface"""

  ANCILLARY_FILES = XenHypervisor.ANCILLARY_FILES + [

    constants.VNC_PASSWORD_FILE,

    ]

  PARAMETERS = {

    constants.HV_ACPI: hv_base.NO_CHECK,

    constants.HV_BOOT_ORDER: (True, ) +

      (lambda x: x and len(x.strip("acdn")) == 0,

       "Invalid boot order specified, must be one or more of [acdn]",

       None, None),

    constants.HV_CDROM_IMAGE_PATH: hv_base.OPT_FILE_CHECK,

    constants.HV_DISK_TYPE:

      hv_base.ParamInSet(True, constants.HT_HVM_VALID_DISK_TYPES),

    constants.HV_NIC_TYPE:

      hv_base.ParamInSet(True, constants.HT_HVM_VALID_NIC_TYPES),

    constants.HV_PAE: hv_base.NO_CHECK,

    constants.HV_VNC_BIND_ADDRESS:

      (False, netutils.IP4Address.IsValid,

       "VNC bind address is not a valid IP address", None, None),

    constants.HV_KERNEL_PATH: hv_base.REQ_FILE_CHECK,

    constants.HV_DEVICE_MODEL: hv_base.REQ_FILE_CHECK,

    constants.HV_VNC_PASSWORD_FILE: hv_base.REQ_FILE_CHECK,

    constants.HV_MIGRATION_PORT: hv_base.REQ_NET_PORT_CHECK,

    constants.HV_MIGRATION_MODE: hv_base.MIGRATION_MODE_CHECK,

    constants.HV_USE_LOCALTIME: hv_base.NO_CHECK,

    # TODO: Add a check for the blockdev prefix (matching [a-z:] or similar).

    constants.HV_BLOCKDEV_PREFIX: hv_base.NO_CHECK,

    constants.HV_REBOOT_BEHAVIOR:

      hv_base.ParamInSet(True, constants.REBOOT_BEHAVIORS),

    constants.HV_CPU_MASK: hv_base.OPT_MULTI_CPU_MASK_CHECK,

    }

  @classmethod

  def _WriteConfigFile(cls, instance, block_devices):

    """Create a Xen 3.1 HVM config file.

    """

    hvp = instance.hvparams

    config = StringIO()

    config.write("# this is autogenerated by Ganeti, please do not edit\n#\n")

    # kernel handling

    kpath = hvp[constants.HV_KERNEL_PATH]

    config.write("kernel = '%s'\n" % kpath)

    config.write("builder = 'hvm'\n")

    config.write("memory = %d\n" % instance.beparams[constants.BE_MEMORY])

    config.write("vcpus = %d\n" % instance.beparams[constants.BE_VCPUS])

    cpu_pinning = cls._CreateConfigCpus(hvp[constants.HV_CPU_MASK])

    if cpu_pinning:

      config.write("%s\n" % cpu_pinning)

    config.write("name = '%s'\n" % instance.name)

    if hvp[constants.HV_PAE]:

      config.write("pae = 1\n")

    else:

      config.write("pae = 0\n")

    if hvp[constants.HV_ACPI]:

      config.write("acpi = 1\n")

    else:

      config.write("acpi = 0\n")

    config.write("apic = 1\n")

    config.write("device_model = '%s'\n" % hvp[constants.HV_DEVICE_MODEL])

    config.write("boot = '%s'\n" % hvp[constants.HV_BOOT_ORDER])

    config.write("sdl = 0\n")

    config.write("usb = 1\n")

    config.write("usbdevice = 'tablet'\n")

    config.write("vnc = 1\n")

    if hvp[constants.HV_VNC_BIND_ADDRESS] is None:

      config.write("vnclisten = '%s'\n" % constants.VNC_DEFAULT_BIND_ADDRESS)

    else:

      config.write("vnclisten = '%s'\n" % hvp[constants.HV_VNC_BIND_ADDRESS])

    if instance.network_port > constants.VNC_BASE_PORT:

      display = instance.network_port - constants.VNC_BASE_PORT

      config.write("vncdisplay = %s\n" % display)

      config.write("vncunused = 0\n")

    else:

      config.write("# vncdisplay = 1\n")

      config.write("vncunused = 1\n")

    vnc_pwd_file = hvp[constants.HV_VNC_PASSWORD_FILE]

    try:

      password = utils.ReadFile(vnc_pwd_file)

    except EnvironmentError, err:

      raise errors.HypervisorError("Failed to open VNC password file %s: %s" %

                                   (vnc_pwd_file, err))

    config.write("vncpasswd = '%s'\n" % password.rstrip())

    config.write("serial = 'pty'\n")

    if hvp[constants.HV_USE_LOCALTIME]:

      config.write("localtime = 1\n")

    vif_data = []

    nic_type = hvp[constants.HV_NIC_TYPE]

    if nic_type is None:

      # ensure old instances don't change

      nic_type_str = ", type=ioemu"

    elif nic_type == constants.HT_NIC_PARAVIRTUAL:

      nic_type_str = ", type=paravirtualized"

    else:

      nic_type_str = ", model=%s, type=ioemu" % nic_type

    for nic in instance.nics:

      nic_str = "mac=%s%s" % (nic.mac, nic_type_str)

      ip = getattr(nic, "ip", None)

      if ip is not None:

        nic_str += ", ip=%s" % ip

      if nic.nicparams[constants.NIC_MODE] == constants.NIC_MODE_BRIDGED:

        nic_str += ", bridge=%s" % nic.nicparams[constants.NIC_LINK]

      vif_data.append("'%s'" % nic_str)

    config.write("vif = [%s]\n" % ",".join(vif_data))

    disk_data = cls._GetConfigFileDiskData(block_devices,

                                           hvp[constants.HV_BLOCKDEV_PREFIX])

    iso_path = hvp[constants.HV_CDROM_IMAGE_PATH]

    if iso_path:

      iso = "'file:%s,hdc:cdrom,r'" % iso_path

      disk_data.append(iso)

    config.write("disk = [%s]\n" % (",".join(disk_data)))

    config.write("on_poweroff = 'destroy'\n")

    if hvp[constants.HV_REBOOT_BEHAVIOR] == constants.INSTANCE_REBOOT_ALLOWED:

      config.write("on_reboot = 'restart'\n")

    else:

      config.write("on_reboot = 'destroy'\n")

    config.write("on_crash = 'restart'\n")

    # just in case it exists

    utils.RemoveFile("/etc/xen/auto/%s" % instance.name)

    try:

      utils.WriteFile("/etc/xen/%s" % instance.name,

                      data=config.getvalue())

    except EnvironmentError, err:

      raise errors.HypervisorError("Cannot write Xen instance confile"

                                   " file /etc/xen/%s: %s" %

                                   (instance.name, err))

    return True