source: vbox/trunk/doc/manual/en_US/user_Troubleshooting.xml@ 33539

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<chapter id="Troubleshooting">
  <title>Troubleshooting</title>

  <para>This chapter provides answers to commonly asked questions. In order to
  improve your user experience with VirtualBox, it is recommended to read this
  section to learn more about common pitfalls and get recommendations on how
  to use the product.</para>

  <sect1>
    <title>Procedures and tools</title>

    <sect2>
      <title>Categorizing and isolating problems</title>

      <para>More often than not, a virtualized guest behaves like a physical
      system. Any problems that a physical machine would encounter, a virtual
      machine will encounter as well. If, for example, Internet connectivity
      is lost due to external issues, virtual machines will be affected just
      as much as physical ones.</para>

      <para>If a true VirtualBox problem is encountered, it helps to
      categorize and isolate the problem first. Here are some of the questions
      that should be answered before reporting a problem:<orderedlist>
          <listitem>
            <para>Is the problem specific to a certain guest OS? Specific
            release of a guest OS? Especially with Linux guest related
            problems, the issue may be specific to a certain distribution and
            version of Linux.</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to a certain host OS? Problems are
            usually not host OS specific (because most of the VirtualBox code
            base is shared across all supported platforms), but especially in
            the areas of networking and USB support, there are significant
            differences between host platforms. Some GUI related issues are
            also host specific.</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to certain host hardware? This
            category of issues is typically related to the host CPU. Because
            of significant differences between VT-x and AMD-V, problems may be
            specific to one or the other technology. The exact CPU model may
            also make a difference (even for software virtualization) because
            different CPUs support different features, which may affect
            certain aspects of guest CPU operation.</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to a certain virtualization mode?
            Some problems may only occur in software virtualization mode,
            others may be specific to hardware virtualization.</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to guest SMP? That is, is it related
            to the number of virtual CPUs (VCPUs) in the guest? Using more
            than one CPU usually significantly affects the internal operation
            of a guest OS.</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to the Guest Additions? In some
            cases, this is a given (e.g., a shared folders problem), in other
            cases it may be less obvious (for example, display problems). And
            if the problem is Guest Additions specific, is it also specific to
            a certain version of the Additions?</para>
          </listitem>

          <listitem>
            <para>Is the problem specific to a certain environment? Some
            problems are related to a particular environment external to the
            VM; this usually involves network setup. Certain configurations of
            external servers such as DHCP or PXE may expose problems which do
            not occur with other, similar servers.</para>
          </listitem>

          <listitem>
            <para>Is the problem a regression? Knowing that an issue is a
            regression usually makes it significantly easier to find the
            solution. In this case, it is crucial to know which version is
            affected and which is not.</para>
          </listitem>
        </orderedlist></para>
    </sect2>

    <sect2>
      <title>Collecting debugging information</title>

      <para>For problem determination, it is often important to collect
      debugging information which can be analyzed by VirtualBox support. This
      section contains information about what kind of information can be
      obtained.</para>

      <para>Every time VirtualBox starts up a VM, a so-called <emphasis
      role="bold">"release log file"</emphasis> is created containing lots of
      information about the VM configuration and runtime events. The log file
      is called <computeroutput><literal>VBox.log</literal></computeroutput>
      and resides in the VM log file folder. Typically this will be a
      directory like this:<screen>$HOME/.VirtualBox/Machines/{machinename}/Logs</screen>When
      starting a VM, the configuration file of the last run will be renamed to
      <computeroutput>.1</computeroutput>, up to
      <computeroutput>.3</computeroutput>. Sometimes when there is a problem,
      it is useful to have a look at the logs. Also when requesting support
      for VirtualBox, supplying the corresponding log file is
      mandatory.</para>

      <para>For convenience, for each virtual machine, the VirtualBox main
      window can show these logs in a window. To access it, select a virtual
      machine from the list on the left and select "Show logs..." from the
      "Machine" window.</para>

      <para>The release log file (VBox.log) contains a wealth of diagnostic
      information, such as Host OS type and version, VirtualBox version and
      build (32-bit or 64-bit), a complete dump of the guest's configuration
      (CFGM), detailed information about the host CPU type and supported
      features, whether hardware virtualization is enabled, information about
      VT-x/AMD-V setup, state transitions (creating, running, paused,
      stopping, etc.), guest BIOS messages, guest Additions messages, device
      specific log entries and at the end of execution, final guest state and
      condensed statistics.</para>

      <para>In case of crashes, it is very important to collect <emphasis
      role="bold">crash dumps</emphasis>. This is true for both host and guest
      crashes. For information about enabling core dumps on Linux, Solaris,
      and OS X systems, refer to the core dump article on the VirtualBox
      website.<footnote>
          <para><ulink
          url="http://www.virtualbox.org/wiki/Core_dump">http://www.virtualbox.org/wiki/Core_dump</ulink>.</para>
        </footnote></para>

      <para>For network related problems, it is often helpful to capture a
      trace of network traffic. If the traffic is routed through an adapter on
      the host, it is possible to use Wireshark or a similar tool to capture
      the traffic there. However, this often also includes a lot of traffic
      unrelated to the VM.</para>

      <para>VirtualBox provides an ability to capture network traffic only on
      a specific VM's network adapter. Refer to the network tracing article on
      the VirtualBox website<footnote>
          <para><ulink
          url="http://www.virtualbox.org/wiki/Network_tips">http://www.virtualbox.org/wiki/Network_tips</ulink>.</para>
        </footnote> for information on enabling this capture. Note that the
      trace files created by VirtualBox are in .pcap format and can be easily
      analyzed with Wireshark.</para>
    </sect2>

    <sect2>
      <title>The built-in VM debugger</title>

      <para>VirtualBox includes a built-in VM debugger, which advanced users
      may find useful. This debugger allows the user to examine, and to some
      extent, control, the VM state.<note>
          <para>Use the VM debugger at your own risk. There is no support for
          it, and the following documentation is only made available for
          advanced users with a very high level of familiarity with the
          x86/AMD64 machine instruction set, as well as detailed knowledge of
          the PC architecture. A degree of familiarity with the internals of
          the guest OS in question is not required, but may be very
          helpful.</para>
        </note></para>

      <para>The VM debugger is available in all regular production versions of
      VirtualBox, but it is disabled by default because the average user will
      have little use for it. There are two ways to access the
      debugger:<itemizedlist>
          <listitem>
            <para>A debugger console window displayed alongside the VM</para>
          </listitem>

          <listitem>
            <para>Via the <computeroutput>telnet</computeroutput> protocol at
            port 5000</para>
          </listitem>
        </itemizedlist></para>

      <para>The debugger can be enabled in three ways:<itemizedlist>
          <listitem>
            <para>Start the <computeroutput>VirtualBox</computeroutput>
            process with a <computeroutput>--dbg</computeroutput>,
            <computeroutput>--debug</computeroutput>, or
            <computeroutput>--debug-command-line</computeroutput> argument.
            See the VirtualBox usage help for details. Note that these
            arguments are only useful when a VM is started immediately, using
            the <computeroutput>--startvm</computeroutput> argument.</para>
          </listitem>

          <listitem>
            <para>Set the
            <computeroutput>VBOX_GUI_DBG_ENABLED</computeroutput> or
            <computeroutput>VBOX_GUI_DBG_AUTO_SHOW</computeroutput>
            environment variable to <computeroutput>true</computeroutput>
            before launching the VirtualBox process. Setting these variables
            (only their presence is checked) is effective even when the first
            VirtualBox process is the VM selector window. VMs subsequently
            launched from the selector will have the debugger enabled.</para>
          </listitem>

          <listitem>
            <para>Set the <computeroutput>GUI/Dbg/Enabled</computeroutput>
            extra data item to <computeroutput>true</computeroutput> before
            launching the VM. This can be set globally or on a per VM
            basis.</para>
          </listitem>
        </itemizedlist></para>

      <para>A new 'Debug' menu entry will be added to the VirtualBox
      application. This menu allows the user to open the debugger
      console.</para>

      <para>The VM debugger command syntax is loosely modeled on Microsoft and
      IBM debuggers used on DOS, OS/2 and Windows. Users familiar with symdeb,
      CodeView, or the OS/2 kernel debugger will find the VirtualBox VM
      debugger familiar.</para>

      <para>The most important command is
      <computeroutput>help</computeroutput>. This will print brief usage help
      for all debugger commands. The set of commands supported by the VM
      debugger changes frequently and the
      <computeroutput>help</computeroutput> command is always
      up-to-date.</para>

      <para>A brief summary of frequently used commands follows:<itemizedlist>
          <listitem>
            <para><computeroutput>stop</computeroutput> -- stops the VM
            execution and enables single stepping</para>
          </listitem>

          <listitem>
            <para><computeroutput>g</computeroutput> -- continue VM
            execution</para>
          </listitem>

          <listitem>
            <para><computeroutput>t</computeroutput> -- single step an
            instruction</para>
          </listitem>

          <listitem>
            <para><computeroutput>rg/rh/r</computeroutput> -- print the
            guest/hypervisor/current registers</para>
          </listitem>

          <listitem>
            <para><computeroutput>kg/kh/k</computeroutput> -- print the
            guest/hypervisor/current call stack</para>
          </listitem>

          <listitem>
            <para><computeroutput>da/db/dw/dd/dq</computeroutput> -- print
            memory contents as ASCII/bytes/words/dwords/qwords</para>
          </listitem>

          <listitem>
            <para><computeroutput>u</computeroutput> -- unassemble
            memory</para>
          </listitem>

          <listitem>
            <para><computeroutput>dg</computeroutput> -- print the guest's
            GDT</para>
          </listitem>

          <listitem>
            <para><computeroutput>di</computeroutput> -- print the guest's
            IDT</para>
          </listitem>

          <listitem>
            <para><computeroutput>dl</computeroutput> -- print the guest's
            LDT</para>
          </listitem>

          <listitem>
            <para><computeroutput>dt</computeroutput> -- print the guest's
            TSS</para>
          </listitem>

          <listitem>
            <para><computeroutput>dp*</computeroutput> -- print the guest's
            page table structures</para>
          </listitem>

          <listitem>
            <para><computeroutput>bp/br</computeroutput> -- set a
            normal/recompiler breakpoint</para>
          </listitem>

          <listitem>
            <para><computeroutput>bl</computeroutput> -- list
            breakpoints</para>
          </listitem>

          <listitem>
            <para><computeroutput>bc</computeroutput> -- clear a
            breakpoint</para>
          </listitem>

          <listitem>
            <para><computeroutput>writecore</computeroutput> -- writes a VM
            core file to disk, refer <xref linkend="guestcoreformat" />
            </para>
          </listitem>
        </itemizedlist></para>

      <para>See the built-in <computeroutput>help</computeroutput> for other
      available commands.</para>

      <para>The VM debugger supports symbolic debugging, although symbols for
      guest code are often not available. For Solaris guests, the
      <computeroutput>detect</computeroutput> command automatically determines
      the guest OS version and locates kernel symbols in guest's memory.
      Symbolic debugging is then available. For Linux guests, the
      <computeroutput>detect</computeroutput> commands also determines the
      guest OS version, but there are no symbols in the guest's memory. Kernel
      symbols are available in the file
      <computeroutput>/proc/kallsyms</computeroutput> on Linux guests. This
      file must be copied to the host, for example using
      <computeroutput>scp</computeroutput>. The
      <computeroutput>loadmap</computeroutput> debugger command can be used to
      make the symbol information available to the VM debugger. Note that the
      <computeroutput>kallsyms</computeroutput> file contains the symbols for
      the currently loaded modules; if the guest's configuration changes, the
      symbols will change as well and must be updated.</para>

      <para>For all guests, a simple way to verify that the correct symbols
      are loaded is the <computeroutput>k</computeroutput> command. The guest
      is normally idling and it should be clear from the symbolic information
      that the guest operating system's idle loop is being executed.</para>

      <para>Another group of debugger commands is the set of
      <computeroutput>info</computeroutput> commands. Running
      <computeroutput>info help</computeroutput> provides complete usage
      information. The information commands provide ad-hoc data pertinent to
      various emulated devices and aspects of the VMM. There is no general
      guideline for using the <computeroutput>info</computeroutput> commands,
      the right command to use depends entirely on the problem being
      investigated. Some of the info commands are:<itemizedlist>
          <listitem>
            <para><computeroutput>cfgm</computeroutput> -- print a branch of
            the configuration tree</para>
          </listitem>

          <listitem>
            <para><computeroutput>cpuid</computeroutput> -- display the guest
            CPUID leaves</para>
          </listitem>

          <listitem>
            <para><computeroutput>ioport</computeroutput> -- print registered
            I/O port ranges</para>
          </listitem>

          <listitem>
            <para><computeroutput>mmio</computeroutput> -- print registered
            MMIO ranges</para>
          </listitem>

          <listitem>
            <para><computeroutput>mode</computeroutput> -- print the current
            paging mode</para>
          </listitem>

          <listitem>
            <para><computeroutput>pit</computeroutput> -- print the i8254 PIT
            state</para>
          </listitem>

          <listitem>
            <para><computeroutput>pic</computeroutput> -- print the i8259A PIC
            state</para>
          </listitem>

          <listitem>
            <para><computeroutput>ohci/ehci</computeroutput> -- print a subset
            of the OHCI/EHCI USB controller state</para>
          </listitem>

          <listitem>
            <para><computeroutput>pcnet0</computeroutput> -- print the PCnet
            state</para>
          </listitem>

          <listitem>
            <para><computeroutput>vgatext</computeroutput> -- print the
            contents of the VGA framebuffer formatted as standard text
            mode</para>
          </listitem>

          <listitem>
            <para><computeroutput>timers</computeroutput> -- print all VM
            timers</para>
          </listitem>
        </itemizedlist></para>

      <para>The output of the <computeroutput>info</computeroutput> commands
      generally requires in-depth knowledge of the emulated device and/or
      VirtualBox VMM internals. However, when used properly, the information
      provided can be invaluable.</para>
    </sect2>

    <sect2 id="guestcoreformat">
      <title>VM core format</title>

      <para>VirtualBox uses the 64-bit ELF format for its VM core files. The
      VM core file contain the memory and CPU dumps of the VM and can be
      useful for debugging your guest OS. The 64-bit ELF object format
      specficiation can be obtained here: <literal><ulink
      url="http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf">http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf</ulink></literal>.</para>

      <para>The overall layout of the VM core format is as follows:</para>

      <para><screen>[ ELF 64 Header]
[ Program Header, type PT_NOTE ]
  -&gt; offset to COREDESCRIPTOR
[ Program Header, type PT_LOAD ] - one for each contiguous physical memory range
  -&gt; Memory offset of range
  -&gt; File offset
[ Note Header, type NT_VBOXCORE ]
[ COREDESCRIPTOR ]
  -&gt; Magic
  -&gt; VM core file version
  -&gt; VBox version
  -&gt; Number of vCPUs etc.
[ Note Header, type NT_VBOXCPU ] - one for each vCPU
[ vCPU 1 Note Header ]
  [ CPUMCTX - vCPU 1 dump ]
[ Additional Notes + Data ] - currently unused
[ Memory dump ]</screen></para>

      <para>The memory descriptors contain physical addresses relative to the
      guest and not virtual addresses. Regions of memory such as MMIO regions
      are not included in the core file.</para>

      <para>The relevant data structures and definitions can be found in the
      VirtualBox sources under the following header files:
      <computeroutput>include/VBox/dbgfcorefmt.h</computeroutput>,
      <computeroutput>include/VBox/cpumctx.h</computeroutput> and
      <computeroutput>src/VBox/Runtime/include/internal/ldrELFCommon.h</computeroutput>.</para>

      <para>The VM core file can be inspected using
      <computeroutput>elfdump</computeroutput> and GNU
      <computeroutput>readelf</computeroutput> or other similar
      utilities.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>General</title>

    <sect2 id="configPeriodicFlush">
      <title>Guest shows IDE/SATA errors for file-based images on slow host
      file system</title>

      <para>Occasionally, some host file systems provide very poor writing
      performance and as a consequence cause the guest to time out IDE/SATA
      commands. This is normal behavior and should normally cause no real
      problems, as the guest should repeat commands that have timed out.
      However some guests (e.g. some Linux versions) have severe problems if a
      write to an image file takes longer than about 15 seconds. Some file
      systems however require more than a minute to complete a single write,
      if the host cache contains a large amount of data that needs to be
      written.</para>

      <para>The symptom for this problem is that the guest can no longer
      access its files during large write or copying operations, usually
      leading to an immediate hang of the guest.</para>

      <para>In order to work around this problem (the true fix is to use a
      faster file system that doesn't exhibit such unacceptable write
      performance), it is possible to flush the image file after a certain
      amount of data has been written. This interval is normally infinite, but
      can be configured individually for each disk of a VM.</para>

      <para>For IDE disks use the following command:</para>

      <screen>VBoxManage setextradata "VM name"
      "VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/FlushInterval" [b]</screen>

      <para>For SATA disks use the following command:</para>

      <screen>VBoxManage setextradata "VM name"
      "VBoxInternal/Devices/ahci/0/LUN#[x]/Config/FlushInterval" [b]</screen>

      <para>The value [x] that selects the disk for IDE is 0 for the master
      device on the first channel, 1 for the slave device on the first
      channel, 2 for the master device on the second channel or 3 for the
      master device on the second channel. For SATA use values between 0 and
      29. Only disks support this configuration option; it must not be set for
      CD/DVD drives.</para>

      <para>The unit of the interval [b] is the number of bytes written since
      the last flush. The value for it must be selected so that the occasional
      long write delays do not occur. Since the proper flush interval depends
      on the performance of the host and the host filesystem, finding the
      optimal value that makes the problem disappear requires some
      experimentation. Values between 1000000 and 10000000 (1 to 10 megabytes)
      are a good starting point. Decreasing the interval both decreases the
      probability of the problem and the write performance of the guest.
      Setting the value unnecessarily low will cost performance without
      providing any benefits. An interval of 1 will cause a flush for each
      write operation and should solve the problem in any case, but has a
      severe write performance penalty.</para>

      <para>Providing a value of 0 for [b] is treated as an infinite flush
      interval, effectively disabling this workaround. Removing the extra data
      key by specifying no value for [b] has the same effect.</para>
    </sect2>

    <sect2>
      <title>Responding to guest IDE/SATA flush requests</title>

      <para>If desired, the virtual disk images can be flushed when the guest
      issues the IDE FLUSH CACHE command. Normally these requests are ignored
      for improved performance. The parameters below are only accepted for
      disk drives. They must not be set for DVD drives.</para>

      <para>To enable flushing for IDE disks, issue the following
      command:</para>

      <screen>VBoxManage setextradata "VM name" "VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/IgnoreFlush" 0</screen>

      <para>The value [x] that selects the disk is 0 for the master device on
      the first channel, 1 for the slave device on the first channel, 2 for
      the master device on the second channel or 3 for the master device on
      the second channel.</para>

      <para>To enable flushing for SATA disks, issue the following
      command:</para>

      <screen>VBoxManage setextradata "VM name" "VBoxInternal/Devices/ahci/0/LUN#[x]/Config/IgnoreFlush" 0</screen>

      <para>The value [x] that selects the disk can be a value between 0 and
      29.</para>

      <para>Note that this doesn't affect the flushes performed according to
      the configuration described in <xref linkend="configPeriodicFlush"
      xrefstyle="template: %n" />. Restoring the default of ignoring flush
      commands is possible by setting the value to 1 or by removing the
      key.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Windows guests</title>

    <sect2>
      <title>Windows bluescreens after changing VM configuration</title>

      <para>Changing certain virtual machine settings can cause Windows guests
      to fail during start up with a bluescreen. This may happen if you change
      VM settings after installing Windows, or if you copy a disk image with
      an already installed Windows to a newly created VM which has settings
      that differ from the original machine.</para>

      <para>This applies in particular to the following settings:<itemizedlist>
          <listitem>
            <para>The ACPI and I/O APIC settings should never be changed after
            installing Windows. Depending on the presence of these hardware
            features, the Windows installation program chooses special kernel
            and device driver versions and will fail to startup should these
            hardware features be removed. (Enabling them for a Windows VM
            which was installed without them does not cause any harm. However,
            Windows will not use these features in this case.)</para>
          </listitem>

          <listitem>
            <para>Changing the storage controller hardware will cause bootup
            failures as well. This might also apply to you if you copy a disk
            image from an older version of VirtualBox to a virtual machine
            created with a newer VirtualBox version; the default subtype of
            IDE controller hardware was changed from PIIX3 to PIIX4 with
            VirtualBox 2.2. Make sure these settings are identical.</para>
          </listitem>
        </itemizedlist></para>
    </sect2>

    <sect2>
      <title>Windows 0x101 bluescreens with SMP enabled (IPI timeout)</title>

      <para>If a VM is configured to have more than one processor (symmetrical
      multiprocessing, SMP), some configurations of Windows guests crash with
      an 0x101 error message, indicating a timeout for inter-processor
      interrupts (IPIs). These interrupts synchronize memory management
      between processors.</para>

      <para>According to Microsoft, this is due to a race condition in
      Windows. A hotfix is available.<footnote>
          <para>See <ulink
          url="http://support.microsoft.com/kb/955076">http://support.microsoft.com/kb/955076</ulink>.</para>
        </footnote> If this does not help, please reduce the number of virtual
      processors to 1.</para>
    </sect2>

    <sect2>
      <title>Windows 2000 installation failures</title>

      <para>When installing Windows 2000 guests, you might run into one of the
      following issues:</para>

      <itemizedlist>
        <listitem>
          <para>Installation reboots, usually during component
          registration.</para>
        </listitem>

        <listitem>
          <para>Installation fills the whole hard disk with empty log
          files.</para>
        </listitem>

        <listitem>
          <para>Installation complains about a failure installing
          <literal>msgina.dll</literal>.</para>
        </listitem>
      </itemizedlist>

      <para>These problems are all caused by a bug in the hard disk driver of
      Windows 2000. After issuing a hard disk request, there is a race
      condition in the Windows driver code which leads to corruption if the
      operation completes too fast, i.e. the hardware interrupt from the IDE
      controller arrives too soon. With physical hardware, there is a
      guaranteed delay in most systems so the problem is usually hidden there
      (however it should be possible to reproduce it on physical hardware as
      well). In a virtual environment, it is possible for the operation to be
      done immediately (especially on very fast systems with multiple CPUs)
      and the interrupt is signaled sooner than on a physical system. The
      solution is to introduce an artificial delay before delivering such
      interrupts. This delay can be configured for a VM using the following
      command:</para>

      <screen>VBoxManage setextradata "VM name" "VBoxInternal/Devices/piix3ide/0/Config/IRQDelay" 1</screen>

      <para>This sets the delay to one millisecond. In case this doesn't help,
      increase it to a value between 1 and 5 milliseconds. Please note that
      this slows down disk performance. After installation, you should be able
      to remove the key (or set it to 0).</para>
    </sect2>

    <sect2>
      <title>How to record bluescreen information from Windows guests</title>

      <para>When Windows guests run into a kernel crash, they display the
      infamous bluescreen. Depending on how Windows is configured, the
      information will remain on the screen until the machine is restarted or
      it will reboot automatically. During installation, Windows is usually
      configured to reboot automatically. With automatic reboots, there is no
      chance to record the bluescreen information which might be important for
      problem determination.</para>

      <para>VirtualBox provides a method of halting a guest when it wants to
      perform a reset. In order to enable this feature, issue the following
      command:</para>

      <para><screen>VBoxManage setextradata "VM name" "VBoxInternal/PDM/HaltOnReset" 1</screen></para>
    </sect2>

    <sect2>
      <title>No networking in Windows Vista guests</title>

      <para>Unfortunately, with Vista, Microsoft dropped support for the
      virtual AMD PCNet card that we are providing to virtual machines. As a
      result, after installation, Vista guests initially have no networking.
      VirtualBox therefore ships a driver for that card with the Windows Guest
      Additions; see <xref linkend="vista_networking" />.</para>

      <para>Starting with version 1.6.0 VirtualBox can emulate an Intel E1000
      network device which is supported by Vista without any third-party
      drivers.</para>
    </sect2>

    <sect2>
      <title>Windows guests may cause a high CPU load</title>

      <para>Several background applications of Windows guests, especially
      virus scanners, are known to increases the CPU load notably even if the
      guest appears to be idle. We recommend to deactivate virus scanners
      within virtualized guests if possible.</para>
    </sect2>

    <sect2 id="win7_audio">
      <title>No audio in Windows Vista (64-bit) and Windows 7 guests</title>

      <para>32-bit Windows 7 does not ship with drivers for our emulated audio
      hardware (AC'97). However, running Windows Update should solve the
      problem by getting an appropriate driver for it automatically. After
      that update followed by a reboot you should have working audio.</para>

      <para>For the 64-bit versions of Windows Vista and 7 you have to
      download the Realtek AC'97 drivers to enable audio.</para>

      <para>See <ulink
      url="http://www.realtek.com.tw/downloads">http://www.realtek.com.tw/downloads</ulink>
      for download instructions.</para>
    </sect2>

    <sect2>
      <title>Long delays when accessing shared folders</title>

      <para>The performance for accesses to shared folders from a Windows
      guest might be decreased due to delays during the resolution of the
      VirtualBox shared folders name service. To fix these delays, add the
      following entries to the file
      <computeroutput>\windows\system32\drivers\etc\lmhosts</computeroutput>
      of the Windows guest:</para>

      <screen>255.255.255.255        VBOXSVR #PRE
255.255.255.255        VBOXSRV #PRE</screen>

      <para>After doing this change, a reboot of the guest is required.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Linux and X11 guests</title>

    <sect2>
      <title>Linux guests may cause a high CPU load</title>

      <para>Some Linux guests may cause a high CPU load even if the guest
      system appears to be idle. This can be caused by a high timer frequency
      of the guest kernel. Some Linux distributions, for example Fedora, ship
      a Linux kernel configured for a timer frequency of <emphasis
      role="bold"> 1000Hz</emphasis>. We recommend to recompile the guest
      kernel and to select a timer frequency of 100Hz.</para>

      <para>Linux kernels shipped with Red Hat Enterprise Linux (RHEL) as of
      release 4.7 and 5.1 as well as kernels of related Linux distributions
      (for instance CentOS and Oracle Enterprise Linux) support a kernel
      parameter <emphasis>divider=N</emphasis>. Hence, such kernels support a
      lower timer frequency without recompilation. We suggest to add the
      kernel parameter <emphasis>divider=10</emphasis> to select a guest
      kernel timer frequency of 100Hz.</para>
    </sect2>

    <sect2>
      <title>AMD Barcelona CPUs</title>

      <para>Most Linux-based guests will fail with AMD Phenoms or
      Barcelona-level Opterons due to a bug in the Linux kernel. Enable the
      I/O-APIC to work around the problem (see <xref
      linkend="settings-general-advanced" />).</para>
    </sect2>

    <sect2 id="trouble-linux-buggy">
      <title>Buggy Linux 2.6 kernel versions</title>

      <para>The following bugs in Linux kernels prevent them from executing
      correctly in VirtualBox, causing VM boot crashes:<itemizedlist>
          <listitem>
            <para>The Linux kernel version 2.6.18 (and some 2.6.17 versions)
            introduced a race condition that can cause boot crashes in
            VirtualBox. Please use a kernel version 2.6.19 or later.</para>
          </listitem>

          <listitem>
            <para>With hardware virtualization and the I/O APIC enabled,
            kernels before 2.6.24-rc6 may panic on boot with the following
            message:<screen>Kernel panic - not syncing: IO-APIC + timer doesn't work!  Boot with
apic=debug and send a report.  Then try booting with the 'noapic' option</screen></para>

            <para>If you see this message, either disable hardware
            virtualization or the I/O APIC (see <xref
            linkend="settings-system" />), or upgrade the guest to a newer
            kernel.<footnote>
                <para>See <ulink
                url="http://www.mail-archive.com/[email protected]/msg30813.html">http://www.mail-archive.com/[email protected]/msg30813.html</ulink>
                for details about the kernel fix.</para>
              </footnote></para>
          </listitem>
        </itemizedlist></para>
    </sect2>

    <sect2>
      <title>Shared clipboard, auto-resizing and seamless desktop in X11
      guests</title>

      <para>Guest desktop services in guests running the X11 window system
      (Solaris, Linux and others) are provided by a guest service called
      <computeroutput>VBoxClient</computeroutput>, which runs under the ID of
      the user who started the desktop session and is automatically started
      using the following command lines <screen>VBoxClient --clipboard
VBoxClient --display
VBoxClient --seamless</screen> when your X11 user session is started if you
      are using a common desktop environment (Gnome, KDE and others). If a
      particular desktop service is not working correctly, it is worth
      checking whether the process which should provide it is running.</para>

      <para>The <computeroutput>VBoxClient</computeroutput> processes create
      files in the user's home directory with names of the form
      <computeroutput>.vboxclient-*.pid</computeroutput> when they are running
      in order to prevent a given service from being started twice. It can
      happen due to misconfiguration that these files are created owned by
      root and not deleted when the services are stopped, which will prevent
      them from being started in future sessions. If the services cannot be
      started, you may wish to check whether these files still exist.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Windows hosts</title>

    <sect2>
      <title>VBoxSVC out-of-process COM server issues</title>

      <para>VirtualBox makes use of the Microsoft Component Object Model (COM)
      for inter- and intra-process communication. This allows VirtualBox to
      share a common configuration among different virtual machine processes
      and provide several user interface options based on a common
      architecture. All global status information and configuration is
      maintained by the process <computeroutput>VBoxSVC.exe</computeroutput>,
      which is an out-of-process COM server. Whenever a VirtualBox process is
      started, it requests access to the COM server and Windows automatically
      starts the process. Note that it should never be started by the end
      user.</para>

      <para>When the last process disconnects from the COM server, it will
      terminate itself after some seconds. The VirtualBox configuration (XML
      files) is maintained and owned by the COM server and the files are
      locked whenever the server runs.</para>

      <para>In some cases - such as when a virtual machine is terminated
      unexpectedly - the COM server will not notice that the client is
      disconnected and stay active for a longer period (10 minutes or so)
      keeping the configuration files locked. In other rare cases the COM
      server might experience an internal error and subsequently other
      processes fail to initialize it. In these situations, it is recommended
      to use the Windows task manager to kill the process
      <computeroutput>VBoxSVC.exe</computeroutput>.</para>
    </sect2>

    <sect2>
      <title>CD/DVD changes not recognized</title>

      <para>In case you have assigned a physical CD/DVD drive to a guest and
      the guest does not notice when the medium changes, make sure that the
      Windows media change notification (MCN) feature is not turned off. This
      is represented by the following key in the Windows registry:<screen><literal>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Cdrom\Autorun</literal></screen>Certain
      applications may disable this key against Microsoft's advice. If it is
      set to 0, change it to 1 and reboot your system. VirtualBox relies on
      Windows notifying it of media changes.</para>
    </sect2>

    <sect2>
      <title>Sluggish response when using Microsoft RDP client</title>

      <para>If connecting to a Virtual Machine via the Microsoft RDP client
      (called Remote Desktop Connection), there can be large delays between
      input (moving the mouse over a menu is the most obvious situation) and
      output. This is because this RDP client collects input for a certain
      time before sending it to the VRDP server built into VirtualBox.</para>

      <para>The interval can be decreased by setting a Windows registry key to
      smaller values than the default of 100. The key does not exist initially
      and must be of type DWORD. The unit for its values is milliseconds.
      Values around 20 are suitable for low-bandwidth connections between the
      RDP client and server. Values around 4 can be used for a gigabit
      Ethernet connection. Generally values below 10 achieve a performance
      that is very close to that of the local input devices and screen of the
      host on which the Virtual Machine is running.</para>

      <para>Depending whether the setting should be changed for an individual
      user or for the system, either</para>

      <screen>HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Min Send Interval</screen>

      <para>or</para>

      <screen>HKEY_LOCAL_MACHINE\Software\Microsoft\Terminal Server Client\Min Send Interval</screen>

      <para>can be set appropriately.</para>
    </sect2>

    <sect2>
      <title>Running an iSCSI initiator and target on a single system</title>

      <para>Deadlocks can occur on a Windows host when attempting to access an
      iSCSI target running in a guest virtual machine with an iSCSI initiator
      (e.g. Microsoft iSCSI Initiator) that is running on the host. This is
      caused by a flaw in the Windows cache manager component, and causes
      sluggish host system response for several minutes, followed by a
      "Delayed Write Failed" error message in the system tray or in a separate
      message window. The guest is blocked during that period and may show
      error messages or become unstable.</para>

      <para>Setting the environment variable
      <computeroutput>VBOX_DISABLE_HOST_DISK_CACHE</computeroutput> to 1 will
      enable a workaround for this problem until Microsoft addresses the
      issue. For example, open a command prompt window and start VirtualBox
      like this:</para>

      <screen>set VBOX_DISABLE_HOST_DISK_CACHE=1
VirtualBox</screen>

      <para>While this will decrease guest disk performance (especially
      writes), it does not affect the performance of other applications
      running on the host.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Linux hosts</title>

    <sect2 id="linuxkernelmodulefailstoload">
      <title>Linux kernel module refuses to load</title>

      <para>If the VirtualBox kernel module
      (<computeroutput>vboxdrv</computeroutput>) refuses to load, i.e. you get
      an "Error inserting vboxdrv: Invalid argument", check (as root) the
      output of the <computeroutput>dmesg</computeroutput> command to find out
      why the load failed. The most common reasons are:</para>

      <itemizedlist>
        <listitem>
          <para>With Linux 2.6.19 and higher, the NMI watchdog may be active.
          Add <computeroutput>nmi_watchdog=0</computeroutput> to the kernel
          command line (e.g. in your grub configuration) and reboot. With the
          Debian and Ubuntu installation modules, execute <computeroutput>sudo
          dpkg-reconfigure virtualbox</computeroutput> again.</para>
        </listitem>

        <listitem>
          <para>The kernel disagrees about the version of the gcc used to
          compile the module. Make sure that you use the same compiler as used
          to build the kernel.</para>
        </listitem>
      </itemizedlist>
    </sect2>

    <sect2>
      <title>Linux host CD/DVD drive not found</title>

      <para>If you have configured a virtual machine to use the host's CD/DVD
      drive, but this does not appear to work, make sure that the current user
      has permission to access the corresponding Linux device file
      (<computeroutput>/dev/hdc</computeroutput> or
      <computeroutput>/dev/scd0</computeroutput> or
      <computeroutput>/dev/cdrom</computeroutput> or similar). On most
      distributions, the user must be added to a corresponding group (usually
      called <computeroutput>cdrom</computeroutput> or
      <computeroutput>cdrw</computeroutput>).</para>
    </sect2>

    <sect2>
      <title>Linux host CD/DVD drive not found (older distributions)</title>

      <para>On older Linux distributions, if your CD/DVD device has a
      different name, VirtualBox may be unable to find it. On older Linux
      hosts, VirtualBox performs the following steps to locate your CD/DVD
      drives:</para>

      <para><orderedlist>
          <listitem>
            <para>VirtualBox examines if the environment variable
            <computeroutput>VBOX_CDROM</computeroutput> is defined (see
            below). If so, VirtualBox omits all the following checks.</para>
          </listitem>

          <listitem>
            <para>VirtualBox tests if
            <computeroutput>/dev/cdrom</computeroutput> works.</para>
          </listitem>

          <listitem>
            <para>In addition, VirtualBox checks if any CD/DVD drives are
            currently mounted by checking
            <computeroutput>/etc/mtab</computeroutput>.</para>
          </listitem>

          <listitem>
            <para>In addition, VirtualBox checks if any of the entries in
            <computeroutput>/etc/fstab</computeroutput> point to CD/DVD
            devices.</para>
          </listitem>
        </orderedlist></para>

      <para>In other words, you can try to set VBOX_CDROM to contain a list of
      your CD/DVD devices, separated by colons, for example as follows:</para>

      <para><screen>export VBOX_CDROM='/dev/cdrom0:/dev/cdrom1'</screen>On
      modern Linux distributions, VirtualBox uses the hardware abstraction
      layer (hal) to locate CD and DVD hardware.</para>
    </sect2>

    <sect2>
      <title>Linux host floppy not found</title>

      <para>The previous instructions (for CD and DVD drives) apply
      accordingly to floppy disks, except that on older distributions
      VirtualBox tests for <computeroutput>/dev/fd*</computeroutput> devices
      by default, and this can be overridden with the
      <computeroutput>VBOX_FLOPPY</computeroutput> environment
      variable.</para>
    </sect2>

    <sect2>
      <title>Strange guest IDE error messages when writing to CD/DVD</title>

      <para>If the experimental CD/DVD writer support is enabled with an
      incorrect VirtualBox, host or guest configuration, it is possible that
      any attempt to access the CD/DVD writer fails and simply results in
      guest kernel error messages (for Linux guests) or application error
      messages (for Windows guests). VirtualBox performs the usual consistency
      checks when a VM is powered up (in particular it aborts with an error
      message if the device for the CD/DVD writer is not writable by the user
      starting the VM), but it cannot detect all misconfigurations. The
      necessary host and guest OS configuration is not specific for
      VirtualBox, but a few frequent problems are listed here which occurred
      in connection with VirtualBox.</para>

      <para>Special care must be taken to use the correct device. The
      configured host CD/DVD device file name (in most cases
      <literal>/dev/cdrom</literal>) must point to the device that allows
      writing to the CD/DVD unit. For CD/DVD writer units connected to a SCSI
      controller or to a IDE controller that interfaces to the Linux SCSI
      subsystem (common for some SATA controllers), this must refer to the
      SCSI device node (e.g. <literal>/dev/scd0</literal>). Even for IDE
      CD/DVD writer units this must refer to the appropriate SCSI CD-ROM
      device node (e.g. <literal>/dev/scd0</literal>) if the
      <literal>ide-scsi</literal> kernel module is loaded. This module is
      required for CD/DVD writer support with all Linux 2.4 kernels and some
      early 2.6 kernels. Many Linux distributions load this module whenever a
      CD/DVD writer is detected in the system, even if the kernel would
      support CD/DVD writers without the module. VirtualBox supports the use
      of IDE device files (e.g. <literal>/dev/hdc</literal>), provided the
      kernel supports this and the <literal>ide-scsi</literal> module is not
      loaded.</para>

      <para>Similar rules (except that within the guest the CD/DVD writer is
      always an IDE device) apply to the guest configuration. Since this setup
      is very common, it is likely that the default configuration of the guest
      works as expected.</para>
    </sect2>

    <sect2>
      <title>VBoxSVC IPC issues</title>

      <para>On Linux, VirtualBox makes use of a custom version of Mozilla
      XPCOM (cross platform component object model) for inter- and
      intra-process communication (IPC). The process
      <computeroutput>VBoxSVC</computeroutput> serves as a communication hub
      between different VirtualBox processes and maintains the global
      configuration, i.e. the XML database. When starting a VirtualBox
      component, the processes <computeroutput>VBoxSVC</computeroutput> and
      <computeroutput>VirtualBoxXPCOMIPCD</computeroutput> are started
      automatically. They are only accessible from the user account they are
      running under. <computeroutput>VBoxSVC</computeroutput> owns the
      VirtualBox configuration database which normally resides in
      <computeroutput>~/.VirtualBox</computeroutput>. While it is running, the
      configuration files are locked. Communication between the various
      VirtualBox components and <computeroutput>VBoxSVC</computeroutput> is
      performed through a local domain socket residing in
      <computeroutput>/tmp/.vbox-&lt;username&gt;-ipc</computeroutput>. In
      case there are communication problems (i.e. a VirtualBox application
      cannot communicate with <computeroutput>VBoxSVC</computeroutput>),
      terminate the daemons and remove the local domain socket
      directory.</para>
    </sect2>

    <sect2 id="usb_linux">
      <title>USB not working</title>

      <para>If USB is not working on your Linux host, make sure that the
      current user is a member of the
      <computeroutput>vboxusers</computeroutput> group. On older hosts, you
      need to make sure that the user has permission to access the USB
      filesystem (<computeroutput>usbfs</computeroutput>), which VirtualBox
      relies on to retrieve valid information about your host's USB devices.
      The rest of this section only applies to those older systems.</para>

      <note>
        <para>The current rdesktop-vrdp implementation does not support
        accessing USB devices through the
        <computeroutput>sysfs</computeroutput>!</para>
      </note>

      <para>As <computeroutput>usbfs</computeroutput> is a virtual filesystem,
      a <computeroutput>chmod</computeroutput> on
      <computeroutput>/proc/bus/usb</computeroutput> has no effect. The
      permissions for <computeroutput>usbfs</computeroutput> can therefore
      <emphasis>only</emphasis> be changed by editing the
      <computeroutput>/etc/fstab</computeroutput> file.</para>

      <para>For example, most Linux distributions have a user group called
      <computeroutput>usb</computeroutput> or similar, of which the current
      user must be a member. To give all users of that group access to usbfs,
      make sure the following line is present:<screen># 85 is the USB group
none     /proc/bus/usb     usbfs      devgid=85,devmode=664    0    0</screen>Replace
      85 with the group ID that matches your system (search
      <computeroutput>/etc/group</computeroutput> for "usb" or similar).
      Alternatively, if you don't mind the security hole, give all users
      access to USB by changing "664" to "666".</para>

      <para>The various distributions are very creative from which script the
      <computeroutput>usbfs</computeroutput> filesystem is mounted. Sometimes
      the command is hidden in unexpected places. For SuSE 10.0 the mount
      command is part of the udev configuration file
      <computeroutput>/etc/udev/rules.d/50-udev.rules</computeroutput>. As
      this distribution has no user group called
      <computeroutput>usb</computeroutput>, you may e.g. use the
      <computeroutput>vboxusers</computeroutput> group which was created by
      the VirtualBox installer. Since group numbers are allocated dynamically,
      the following example uses 85 as a placeholder. Modify the line
      containing (a linebreak has been inserted to improve
      readability)<screen>DEVPATH="/module/usbcore", ACTION=="add",
    RUN+="/bin/mount -t usbfs usbfs /proc/bus/usb"</screen> and add the
      necessary options (make sure that everything is in a single
      line):<screen>DEVPATH="/module/usbcore", ACTION=="add",
    RUN+="/bin/mount -t usbfs usbfs /proc/bus/usb -o devgid=85,devmode=664"</screen></para>

      <para>Debian Etch has the mount command in
      <computeroutput>/etc/init.d/mountkernfs.sh</computeroutput>. Since that
      distribution has no group <computeroutput>usb</computeroutput>, it is
      also the easiest solution to allow all members of the group
      <computeroutput>vboxusers</computeroutput> to access the USB subsystem.
      Modify the line <screen>domount usbfs usbdevfs /proc/bus/usb -onoexec,nosuid,nodev</screen>
      so that it contains <screen>domount usbfs usbdevfs /proc/bus/usb -onoexec,nosuid,nodev,devgid=85,devmode=664</screen>
      As usual, replace the 85 with the actual group number which should get
      access to USB devices.</para>

      <para>Other distributions do similar operations in scripts stored in the
      <computeroutput>/etc/init.d</computeroutput> directory.</para>
    </sect2>

    <sect2>
      <title>PAX/grsec kernels</title>

      <para>Linux kernels including the grsec patch (see <literal><ulink
      url="http://www.grsecurity.net/">http://www.grsecurity.net/</ulink></literal>)
      and derivates have to disable PAX_MPROTECT for the VBox binaries to be
      able to start a VM. The reason is that VBox has to create executable
      code on anonymous memory.</para>
    </sect2>

    <sect2>
      <title>Linux kernel vmalloc pool exhausted</title>

      <para>When running a large number of VMs with a lot of RAM on a Linux
      system (say 20 VMs with 1GB of RAM each), additional VMs might fail to
      start with a kernel error saying that the vmalloc pool is exhausted and
      should be extended. The error message also tells you to specify
      <computeroutput>vmalloc=256MB</computeroutput> in your kernel parameter
      list. If adding this parameter to your GRUB or LILO configuration makes
      the kernel fail to boot (with a weird error message such as "failed to
      mount the root partition"), then you have probably run into a memory
      conflict of your kernel and initial RAM disk. This can be solved by
      adding the following parameter to your GRUB configuration:</para>

      <screen>uppermem 524288</screen>
    </sect2>
  </sect1>

  <sect1>
    <title>Solaris hosts</title>

    <sect2>
      <title>Cannot start VM, not enough contiguous memory</title>

      <para>The ZFS file system is known to use all available RAM as cache if
      the default system settings are not changed. This may lead to a heavy
      fragmentation of the host memory preventing VirtualBox VMs from being
      started. We recommend to limit the ZFS cache by adding a line<screen>set zfs:zfs_arc_max = xxxx</screen>
      to /etc/system where <computeroutput>xxxx</computeroutput> bytes is the
      amount of memory usable for the ZFS cache.</para>
    </sect2>

    <sect2>
      <title>VM aborts with out of memory errors on Solaris 10 hosts</title>

      <para>32-bit Solaris 10 hosts (bug 1225025) require swap space equal to,
      or greater than the host's physical memory size. For example, 8 GB
      physical memory would require at least 8 GB swap. This can be configured
      during a Solaris 10 install by choosing a 'custom install' and changing
      the default partitions.</para>

      <note>
        <para>This restriction applies only to 32-bit Solaris hosts, 64-bit
        hosts are not affected!</para>
      </note>

      <para>For existing Solaris 10 installs, an additional swap image needs
      to be mounted and used as swap. Hence if you have 1 GB swap and 8 GB of
      physical memory, you require to add 7 GB more swap. This can be done as
      follows:</para>

      <para>For ZFS (as root user):</para>

      <para><screen>zfs create -V 8gb /_&lt;ZFS volume&gt;_/swap
swap -a /dev/zvol/dsk/_&lt;ZFS volume&gt;_/swap</screen></para>

      <para>To mount if after reboot, add the following line to
      /etc/vfstab:</para>

      <screen>/dev/zvol/dsk/_&lt;ZFS volume&gt;_/swap - - swap - no -</screen>

      <para>Alternatively, you could grow the existing swap using:</para>

      <screen>zfs set volsize=8G rpool/swap</screen>

      <para>And reboot the system for the changes to take effect.</para>

      <para>For UFS (as root user):</para>

      <screen>mkfile 7g /path/to/swapfile.img
swap -a /path/to/swapfile.img</screen>

      <para>To mount it after reboot, add the following line to
      /etc/vfstab:</para>

      <screen>/path/to/swap.img - - swap - no -</screen>
    </sect2>
  </sect1>
</chapter>