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Virtualization Guide
  1. Preface
  2. I Introduction
    1. 1 Virtualization technology
    2. 2 Virtualization scenarios
    3. 3 Introduction to Xen virtualization
    4. 4 Introduction to KVM virtualization
    5. 5 Virtualization tools
    6. 6 Installation of virtualization components
  3. II Managing virtual machines with libvirt
    1. 7 libvirt daemons
    2. 8 Preparing the VM Host Server
    3. 9 Guest installation
    4. 10 Basic VM Guest management
    5. 11 Connecting and authorizing
    6. 12 Advanced storage topics
    7. 13 Configuring virtual machines with Virtual Machine Manager
    8. 14 Configuring virtual machines with virsh
    9. 15 Xen to KVM migration guide
  4. III Hypervisor-independent features
    1. 16 Disk cache modes
    2. 17 VM Guest clock settings
    3. 18 libguestfs
    4. 19 QEMU guest agent
    5. 20 Software TPM emulator
    6. 21 Creating crash dumps of a VM Guest
  5. IV Managing virtual machines with Xen
    1. 22 Setting up a virtual machine host
    2. 23 Virtual networking
    3. 24 Managing a virtualization environment
    4. 25 Block devices in Xen
    5. 26 Virtualization: configuration options and settings
    6. 27 Administrative tasks
    7. 28 XenStore: configuration database shared between domains
    8. 29 Xen as a high-availability virtualization host
    9. 30 Xen: converting a paravirtual (PV) guest into a fully virtual (FV/HVM) guest
  6. V Managing virtual machines with QEMU
    1. 31 QEMU overview
    2. 32 Setting up a KVM VM Host Server
    3. 33 Guest installation
    4. 34 Running virtual machines with qemu-system-ARCH
    5. 35 Virtual machine administration using QEMU monitor
  7. VI Troubleshooting
    1. 36 Integrated help and package documentation
    2. 37 Gathering system information and logs
  8. Glossary
  9. A Configuring GPU Pass-Through for NVIDIA cards
  10. B GNU licenses
Applies to openSUSE Leap 15.5

17 VM Guest clock settings Edit source


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: 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).

17.1 KVM: using kvm_clock Edit source

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:

> 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:

> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
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.

17.1.1 Other timekeeping methods Edit source

The paravirtualized kvm-clock is currently not for Windows* operating systems. For Windows*, use the Windows Time Service Tools for time synchronization.

17.2 Xen virtual machine clock settings Edit source

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 most hardware and software environments.

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

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