Re: [Qemu-devel] Changing RTC from UTC to local time

[Bartosz Fabianowski <bartosz@fabianowski.de>]

> There is a problem with setting RTC to local time  with  daylight
> savings, when running multiple OSs on the same hardware.

Actually, it's not quite as bad.

First of all, the problem you described affects all multi boot systems. 
If more than one operating system decides to adjust for DST, yes, you 
get a clock that is off. That is a know problem and users of multi boot 
systems, which are quite common these days, have learned to deal with 
it. When you are installing Windows, it suggests you let it adjust for 
DST automatically. However, you can uncheck the relevant check box and 
Windows will leave your RTC alone. So, you can designate one operating 
system to take care of the DST adjustment or you can even do it manually 
if you want to. Again, this is not a new issue uncovered here; it 
affects all multi boot systems.

With QEMU, the situation is actually much better. When you shut down 
QEMU, its RTC state is not saved anywhere and it gets initialized from 
the host's RTC the next time QEMU is run. And also, while it is running, 
QEMU keeps synchronizing its RTC with the host time. So if you run 
multiple OSes inside QEMU and if you allow any of them to adjust for 
DST, this will actually have no effect. Therefore, I don't see a problem 
with allowing the RTC to be running in local time and to allow the 
operating systems to play with it.

> Conclusion : setting RTC to local time is a _broken_ concept.

I agree that the world would be a better - and much simpler - place if 
all RTCs ran in UTC. However, this is simply not the case. PC RTCs have 
historically always run in local time. While some PC servers may be 
handling this differently these days, it is certainly still true of 
desktops. And if I understand the idea behind QEMU right, it is to 
emulate a PC and behave like a real one would. So to follow the vast 
majority of PCs out there, QEMU should have a local time RTC by default.

And after all, with my patch, you would still have the option of using 
an UTC RTC if you choose so.

- Bartosz