Re: [Qemu-devel] Rethinking missed tick catchup

[Gleb Natapov <gleb@redhat.com>]

On Wed, Sep 19, 2012 at 06:34:46PM +0300, Avi Kivity wrote:
> On 09/16/2012 05:37 PM, Anthony Liguori wrote:
> > Avi Kivity <avi@redhat.com> writes:
> > 
> >> On 09/13/2012 09:27 PM, Anthony Liguori wrote:
> >>> If there was a better/equivalent solution that didn't depend on qemu-ga,
> >>> I'd be all for it.  But there isn't AFAICT.
> >>
> >> Perhaps there is.  We fixed the problem for Linux by adding kvmclock and
> >> backporting it to distros that users are most likely to use.  Windows
> >> fixed the problem by adding their own pv clock interface.  So we need to
> >> implement that, then focus on tick catchup for Windows XP and other
> >> guests with no pv interface (*BSD, etc.)
> > 
> > Tick catchup simply isn't going to work.  That's the whole point of the thread.
> 
> I'll restate.  Windows and Linux don't need either qemu-ga or tick
> catchup since they have pv time interfaces.  FreeBSD and less frequently
> used guests are unlikely to get a qemu-ga port, so they need tick
> catchup.  Is there reason to believe tick catchup won't work on FreeBSD?
> 
If FreeBSD tries to compensate for lost ticks it may not work.

> >>
> >> Those older guests are also less likely to have a qemu-ga port or
> >> administrator motivation to install it.
> > 
> > That's a strange assertion to make.  FWIW, the issue with hibernation
> > was reported to me with a combination of WinXP and Windows 7 guests, in
> > this case, it's a totally new deployment.  Adding qemu-ga is totally
> > reasonable.
> 
> Windows 7 doesn't need anything if we implement the pv time interface.
What PV interface exactly? According to [1] Hyper-v also tries to
"catch-up" timer by shortening timer period unless to many events were
missed.

[1] http://msdn.microsoft.com/en-us/library/windows/hardware/ff542561%28v=vs.85%29.aspx

> That is less effort than requiring a qemu-ga installation.  Windows XP
> is an edge case.  We can of course support qemu-ga for it, or we can
> massage the tick code to work with it, since it's timekeeping is likely
> a lot less sophisticated than 7's.
> 
How do you propose to "massage the tick code" to compensate for 100
hours of missed ticks in a sane way? As far as I know there is no
difference in timekeeping between Windows XP and Windows 7 (at least
without PV).

--
			Gleb.