Xen keeps making the time wrong on a Windows guest

Question

Just upgraded our Ubuntu Server to 12.04.1 LTS. It runs a Xen VM, which is Windows Server 2008 R2 x64.

We are using the xl toolchain for Xen.

We are in the UK, which is currently GMT+1 (BST).

The time is correct on the host, i.e. it knows we're in BST.

tf@hostname:~$ date
Mon Oct  1 12:16:45 BST 2012

The time on the Windows guest keeps getting reset to GMT, i.e. 1 hour behind where it should be.

We had this on the previous Ubuntu release using the xm toolchain, which was fixed with the option localtime=1, however xl seems to ignore the localtime option.

I cannot find a mechanism to either:

• get Xen to pass local time through to the guests with the xl toolchain

or

• get Xen to leave the guest clock alone and allow NTP on the guest to keep things in sync.

Answer 1

From http://xen.1045712.n5.nabble.com/Time-is-off-by-an-hour-in-my-XEN-vm-tp2586195p2586196.html

There is a RealTimeIsUniversal registry flag hidden in the windows registry that can be set (its not in by default) to let Windows interpret the RTC as UTC as well.

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation] "RealTimeIsUniversal"=dword:00000001