Applies to openSUSE Leap 15

15 VM Guest Clock Settings

Abstract

Keeping the correct time in a VM Guest is one of the more difficult aspects of virtualization. Keeping the correct time is especially important for network applications and is also a prerequisite to do a live migration of a VM Guest.

Tip
Tip: Timekeeping on the VM Host Server

It is strongly recommended to ensure the VM Host Server keeps the correct time as well, for example, by using NTP (see Book “Reference”, Chapter 18 “Time Synchronization with NTP” for more information).

15.1 KVM: Using kvm_clock

KVM provides a paravirtualized clock which is supported via the kvm_clock driver. It is strongly recommended to use kvm_clock.

Use the following command inside a VM Guest running Linux to check whether the driver kvm_clock has been loaded:

tux > sudo dmesg | grep kvm-clock
[    0.000000] kvm-clock: cpu 0, msr 0:7d3a81, boot clock
[    0.000000] kvm-clock: cpu 0, msr 0:1206a81, primary cpu clock
[    0.012000] kvm-clock: cpu 1, msr 0:1306a81, secondary cpu clock
[    0.160082] Switching to clocksource kvm-clock

To check which clock source is currently used, run the following command in the VM Guest. It should output kvm-clock:

tux > cat /sys/devices/system/clocksource/clocksource0/current_clocksource
Important
Important: kvm-clock and NTP

When using kvm-clock, it is recommended to use NTP in the VM Guest, as well. Using NTP on the VM Host Server is also recommended.

15.1.1 Other Timekeeping Methods

The paravirtualized kvm-clock is currently not for Windows* operating systems. For Windows*, use the Windows Time Service Tools for time synchronization (see http://technet.microsoft.com/en-us/library/cc773263%28WS.10%29.aspx for more information).

15.2 Xen Virtual Machine Clock Settings

With Xen 4, the independent wallclock setting /proc/sys/xen/independent_wallclock used for time synchronization between Xen host and guest was removed. A new configuration option tsc_mode was introduced. It specifies a method of utilizing the timestamp counter to synchronize the guest time with the Xen server. Its default value '0' handles the vast majority of hardware and software environments.

For more details on tsc_mode, see the xen-tscmode manual page (man 7 xen-tscmode).

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