Re: [Xen-devel] [PATCHl] localtime basis for paravirtualized guests

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

I must be missing something. Are you saying we might want to provide a 
domU with a localtime RTC, which it reads, offsets to UTC (because it 
knows its timezone), and then uses to set UTC time in its kernel? How 
is that better than what we do now, which is to simply provide UTC time 
directly to the kernel, which it can then convert to localtime itself 
when that's appropriate?
 -- Keir

On 21 Jun 2006, at 22:23, Bruce Rogers wrote:

> The way Linux gets its UTC time when it is running from a locatime
> time base is that hwclock is called from the init scripts and it reads
> /dev/rtc, offsets that value by the local time offset, and resets the
> kernel's time via the settimeofday system call.  So if /dev/rtc was
> tied straight into the time retrieved from Xen (speaking of the
> DomU case) this all works correctly to reset the kernel's sense
> of time to be UTC.  As it stands today, hwclock fails because
> neither /dev/rtc nor direct access to the RTC via port I/O is
> there for a DomU.
>
> Without the above actions taking place, the kernel is left
> using localtime (from Xen) as if it was UTC, and the
> localtime derived from that is off.  I've got the rtc.c code
> hacked up to make it work but before I proceeded down that
> path further, was interested in hearing what others thought
> might be the best solution for allowing guests to use Xen as
> a time basis, but be more flexible with managing its own time.
>
> - Bruce
>
> > > > On 6/21/2006 at 10:11 AM, in message
> <220a261ede8f0089ecc6a7c408cea2b8@xxxxxxxxxxxx>, Keir Fraser
> <Keir.Fraser@xxxxxxxxxxxx> wrote:
>
> > On 21 Jun 2006, at 00:20, Bruce Rogers wrote:
> >
> > > I should point out however that this by itself does not allow a
> > > localtime
> > > time base to be used for xenolinux.  That support would require
> > > additional
> > > changes to Linux (eg a Xen aware implementation of /dev/rtc &
> etc.),
> > > and
> > > should probably be based on a more flexible and thorough
> implementation
> > > of
> > > non-UTC guest time bases than what this patch provides.
> > I'm not sure what you mean here. If Xen adds an offset to wc_sec then
> > XenLinux will see a different wallclock time. Right? RTC isn't
> emulated
> > or used by domUs.
> >
> >   -- Keir
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