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Chapter 14. KVM Guest Timing Management

Virtualization involves several challenges for time keeping in guest virtual machines.
  • Interrupts cannot always be delivered simultaneously and instantaneously to all guest virtual machines. This is because interrupts in virtual machines are not true interrupts - they are injected into the guest virtual machine by the host machine.
  • The host may be running another guest virtual machine, or a different process. Thereofore, the precise timing typically required by interrupts may not always be possible.
Guest virtual machines without accurate time keeping may experience issues with network applications and processes, as session validity, migration, and other network activities rely on timestamps to remain correct.
KVM avoids these issues by providing guest virtual machines with a paravirtualized clock (kvm-clock). However, it is still important to test timing before attempting activities that may be affected by time keeping inaccuracies, such as guest migration.

Important

To avoid the problems described above, the Network Time Protocol (NTP) should be configured on the host and the guest virtual machines. On guests using Red Hat Enterprise Linux 6 and earlier, NTP is implemented by the ntpd service. For more information, see the Red Hat Enterprise 6 Deployment Guide.

14.1. Constant Time Stamp Counter (TSC)
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Modern Intel and AMD CPUs provide a constant Time Stamp Counter (TSC). The count frequency of the constant TSC does not vary when the CPU core itself changes frequency, for example to comply with a power saving policy. A CPU with a constant TSC frequency is necessary in order to use the TSC as a clock source for KVM guests.
Your CPU has a constant Time Stamp Counter if the constant_tsc flag is present. To determine if your CPU has the constant_tsc flag run the following command: 
$ cat /proc/cpuinfo | grep constant_tsc
Copy to Clipboard Toggle word wrap
If any output is given your CPU has the constant_tsc bit. If no output is given follow the instructions below.
Systems without a constant TSC frequency cannot use the TSC as a clock source for virtual machines, and require additional configuration. Power management features interfere with accurate time keeping and must be disabled for guest virtual machines to accurately keep time with KVM.

Important

These instructions are for AMD revision F CPUs only.
If the CPU lacks the constant_tsc bit, disable all power management features (BZ#513138). 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 (2.6.32-330.x86_64)
        root (hd0,0)
	kernel /vmlinuz-2.6.32-330.x86_64 ro root=/dev/VolGroup00/LogVol00 rhgb quiet \
   processor.max_cstate=1
Copy to Clipboard Toggle word wrap
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.
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