Installation guide

Chapter 14. KVM guest timing management
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If the CPU lacks the constant_tsc bit, disable all power management features (BZ#513138
1
).
Each system has several timers it uses to keep time. The TSC is not stable on the host, which
is sometimes caused by cpufreq changes, deep C state, or migration to a host with a faster
TSC. Deep C sleep states can stop the TSC. To prevent the kernel using deep C states append
processor.max_cstate=1 to the kernel boot options in the grub.conf file on the host:
title Red Hat Enterprise Linux (2.6.32-36.x86-64)
root (hd0,0)
kernel /vmlinuz-2.6.32-36.x86-64 ro root=/dev/VolGroup00/LogVol00 rhgb
quiet processor.max_cstate=1
Disable cpufreq (only necessary on hosts without the constant_tsc) by editing the /etc/
sysconfig/cpuspeed configuration file and change the MIN_SPEED and MAX_SPEED variables
to the highest frequency available. Valid limits can be found in the /sys/devices/system/cpu/
cpu*/cpufreq/scaling_available_frequencies files.
Using the para-virtualized clock with Red Hat Enterprise Linux guests
For certain Red Hat Enterprise Linux guests, additional kernel parameters are required. These
parameters can be set by appending them to the end of the /kernel line in the /boot/grub/grub.conf file
of the guest.
The table below lists versions of Red Hat Enterprise Linux and the parameters required for guests on
systems without a constant Time Stamp Counter.
Red Hat Enterprise Linux Additional guest kernel parameters
6.0 AMD64/Intel 64 with the
para-virtualized clock
Additional parameters are not required
6.0 AMD64/Intel 64 without the
para-virtualized clock
notsc lpj=n
5.5 AMD64/Intel 64 with the
para-virtualized clock
Additional parameters are not required
5.5 AMD64/Intel 64 without the
para-virtualized clock
divider=10 notsc lpj=n
5.5 x86 with the para-virtualized
clock
Additional parameters are not required
5.5 x86 without the para-
virtualized clock
divider=10 clocksource=acpi_pm lpj=n
5.4 AMD64/Intel 64 divider=10 notsc
5.4 x86 divider=10 clocksource=acpi_pm
5.3 AMD64/Intel 64 divider=10 notsc
5.3 x86 divider=10 clocksource=acpi_pm
4.8 AMD64/Intel 64 notsc divider=10
4.8 x86 clock=pmtmr divider=10
3.9 AMD64/Intel 64 Additional parameters are not required
3.9 x86 Additional parameters are not required
1
https://bugzilla.redhat.com/show_bug.cgi?id=513138