19 VM Guest clock settings #
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.
It is strongly recommended to ensure the VM Host Server keeps the correct time as well, for example, by using NTP (see Chapter 30, Time synchronization with NTP for more information).
19.1 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:
>sudodmesg | 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:
> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
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.
19.1.1 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.
19.2 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 using the time stamp 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).