Re: Time is off by an hour in my XEN vm

>
> "Frank Groeneveld" wrote:
>
>>
>> Yes, that will probably work, but it used to work without such  
>> tricks.
>> I'm sure it's just a setting in my VM or something like that.
>
> I think to know what that issue might be, and it is most likely not  
> a Xen Software problem at all, but a general virtualization issue. I  
> had to deal with similar problems in the past as well and collected  
> the following information from google + different other sources.
>
> As you might know, there is a posix standard that defines any  
> computer's RTC (CMOS Real Time Clock) to be UTC (or GMT or however  
> you call it). Anyhow, as you probably also might or might not  
> assume, Microsoft does not follow that standard and interprets the  
> RTC as localtime (GMT +/- X). It shows +1h on your system, so I  
> assume you are GMT+1 (or UTC+1). There are probably reasons for  
> Microsoft doing so, even if we can't really spot them - most likely  
> because of backward compatibility -who knows ;) As you can see -  
> there is a discrepancy between your host, running linux, following  
> posix, and you guest, that gets no own RTC from xen based on the  
> nature of xen virtualization and therefore needs to share the real  
> RTC with linux but the OS is not following posix.
>
> 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
>
> This possibility seems to be there as a leftover from the days when  
> NT still ran on RISC machines with UTC RTCs but is not even official  
> documented anywhere as far as I've seen. So I would not assume that  
> it will be there forever. The RTIU registry key is still active  
> within the NT kernel for at least XP and 2003 and works in a way to  
> tell Windows to interpret the Hardware clock as UTC, BUT - only on  
> boot time (actually the kernel seems to read that key directly,  
> check the NT kernel with "strings"). So you can use that as a  
> workaround whenever you run your linux host posix and windows on top.
>
> I have recognized that windows has another special "gotcha" you need  
> to know regarding the clock that is not well documented. Windows  
> tries to sync time exactly every hour (starting 1h after boot) with  
> the hardware CMOS clock AS LONG AS you either don't have NTP  
> configured or none of the configured NTP server(s) is reachable. In  
> that case, the RealTimeIsUniversal key is not interpreted by the  
> running OS - the reg key seems to be only used by the kernel itsef  
> at boot time - you can find the string within the kernel binary -  
> but nowhere else in the OS. So this may become another issue, even  
> if you set the windows registry key above. As long as NTP sync is  
> configured and any NTP server is reachable on bootime+1h+1h+...,  
> Windows does not sync with CMOS anymore but uses the NTP as time  
> source. As soon as I have a working NTP sync in place, the clock  
> does not get set back to UTC anymore, otherwise it would even get  
> set back to localtime every hour..
>
> Another option would be to set your bios to localtime and tell linux  
> to ignore the posix standard - linux deals much better with that  
> than windows with the counterpart. So you would have to tell the  
> linux xen host to interpret the RTC as local (check http://man.he.net/man8/clock)
> . Most people recommend that when one runs windows and linux on the  
> same host (e.g. within xen environments) as the best practice.
>
> hope this helped
> cheers
> Ralf
>
>
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