Re: [PATCH] tsc: use kvmclock for calibration

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Aug 09, 2012 at 05:20:11PM +0300, Avi Kivity wrote:
> On 08/09/2012 05:18 PM, Gerd Hoffmann wrote:
> >   Hi,
> > 
> >>> So what do you suggest?  The options I see are:
> >>>
> >>>   (1) Use this patch (with alignment issue fixed of course).
> >>>   (2) Do a full kvmclock implementation.  Feels a bit like overkill.
> >>>   (3) SeaBIOS can fallback to the PIT for timing on machines which
> >>>       have no TSC.  We could do that too in case we detect kvm ...
> >> 
> >> What sort of timeouts are these?  If seconds, maybe the rtc would be best.
> > 
> > All sorts of timeouts, from a few miliseconds to seconds.
> > 
> > The problematic ones are the longer timeouts, which wait for I/O stuff
> > like disk reads complete.  The stuff with smaller timeouts (like waiting
> > for AHCI link become ready) tend to finish instantly in kvm.
> 
> That's not guaranteed.  The AHCI adapter might be real hardware.  Or the
> emulation may change.
> 
> What's wrong with having a full kvmclock implementation?  Instead of
> issuing rdtsc call a function pointer.

Its not necessary (someone is going to maintain the kvmclock frequency
retrieve, which patch is already here, versus maintainance of 
full kvmclock).

Frequency scaling (or the software equivalent: TSC trapping) are
required for other reasons anyway.