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14.3. Using the Real-Time Clock with Windows Server 2008, Windows Server 2008 R2, and Windows 7 Guests

Windows uses both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For Windows guest virtual machines the Real-Time Clock can be used instead of the TSC for all time sources, which resolves guest timing issues.
The boot.ini file is no longer used as of Windows Server 2008 and newer. Windows Server 2008, Windows Server 2008 R2, and Windows 7 do not use the TSC as a time source if the hypervisor-present bit is set. The Red Hat Enterprise Linux 6 KVM hypervisor enables this CPUID bit by default, so it is no longer necessary to use the Boot Configuration Data Editor (bcdedit.exe) to modify the Windows boot parameters.

Procedure 14.1. Using the Real-Time Clock with Windows Server 2008 R2 and Windows 7 guests

  1. Open the Windows guest virtual machine.
  2. Open the Accessories menu of the start menu. Right click on the Command Prompt application, select Run as Administrator.
  3. Confirm the security exception, if prompted.
  4. Set the boot manager to use the platform clock. This should instruct Windows to use the PM timer for the primary clock source. The system UUID ({default} in the example below) should be changed if the system UUID is different than the default boot device.
    C:\Windows\system32>bcdedit /set {default} USEPLATFORMCLOCK on
    The operation completed successfully
This fix should improve time keeping for Windows Server 2008 R2 and Windows 7 guests. Windows 2008 (non-R2) does not support the USEPLATFORMCLOCK parameter, but already uses the Real-Time Clock by default.