Installation guide

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.
To enable the Real-Time Clock for the P MT IMER clock source (the P MT IMER usually uses the TSC),
add the following option to the Windows boot settings. Windows boot settings are stored in the
boot.ini file. Add the following option to the end of the Windows boot line in the bo o t. i ni file:
/usepmtimer
For more information on Windows boot settings and the usepmtimer option, refer to Available switch
options for the Windows XP and the Windows Server 2003 Boot.ini files.
14 .4. Using t he Real-T ime Clock wit h Windows Server 2008, Windows
Server 2008 R2, and Windows 7 guest s
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 bo o t. i ni 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 hypervi so r-
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 Bo o t C o n f ig u rat io n Dat a Edit o r (bcd ed i t. exe) to
modify the Windows boot parameters.
Pro ced u re 14 .1. Usin g t he Real- Time Clo ck wit h Win d o ws Server 2008 R2 an d Win d o ws 7
g u est s
1. Open the Windows guest virtual machine.
2. Open the Accessories menu of the st art menu. Right click on the Co mman d Pro mp t
application, select R un as Ad mi ni strato r.
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.
14 .5. St eal t ime account ing
Steal time is the amount of CPU time desired by a guest virtual machine that is not provided by the
host. Steal time occurs when the host allocates these resources elsewhere: for example, to another
guest.
Red Hat Ent erp rise Linux 6 Virt ualiz at ion Host Configurat ion and G uest Inst allat ion G uide
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