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

[Bartosz Fabianowski]

> So, you should probably _always_ have the emulated OS keep away from 
> the RTC.  The host OS will provide the correct time as the fake RTC 
> value.

My point exactly. Your host operating system is most likely one you use
quite a lot so chances are you will have set it up correctly to take
care of DST adjustments. When installing a slave OS that requires a
local RTC (Windows), you can just tell it the RTC is local and also tell
it to keep away from the RTC. When the host adjusts for DST, the slave
will automatically follow - theoretically, this should work even if it
occurs while QEMU is running. Though I haven't tried this yet.

And as a further safeguard, as I pointed out earlier, QEMU will always
reset the RTC if the slave OS starts tinkering with it. So even if you
forget to tell your slave OS to stop setting the RTC, no harm is done.

> Actually, while installing SuSE 9.1 yesterday, I noted that it states
> that the clock should be set to UTC unless you are dual booting.

I have never installed Linux on a machine myself so I didn't know what
Linux recommends. From your post and my own experience I conclude once
again that both Linux and *BSD can deal with a local time RTC while
Windows actually requires one. So if your RTC is running in local time,
all of those OSes will work. However, when your RTC is UTC, only Linux
and *BSD will work correctly. Windows will give you the wrong time. This
is why I believe we should have:

a) a choice between UTC and local time and
b) a safe default that makes all OSes work - which is local time

I guess it breaks down to this: If you are tinkering with Linux or *BSD
inside QEMU, you want your RTC to be UTC, as that is technically the
better way of doing it and it works nicely. If you are installing
Windows however, you want local time. Judging from the current posts on
the mailing list, the number of people who install Windows for now seems
to be smaller than that of people who install Linux or *BSD. But I could
well imagine that once QEMU matures and the word gets out, people will
start using it as a VMWare replacement for when they need that lonely
Windows application and Wine just won't do. Which happens to be the
situation I am in. So in anticipation of such a use - a use by newbies -
I believe a safe default is very important.

Thus, again, I believe my patch should be committed. But that, after
all, is Fabrice's choice.

- Bartosz