Re: [Qemu-devel] [PATCH v3 5/5] hpet 'driftfix': add code in hpet_timer() to compensate delayed callbacks and coalesced interrupts

[Gleb Natapov]

On Wed, May 04, 2011 at 10:36:12AM -0300, Glauber Costa wrote:
> On Wed, 2011-05-04 at 06:09 -0300, Marcelo Tosatti wrote:
> > On Wed, May 04, 2011 at 04:06:59AM -0400, Ulrich Obergfell wrote:
> > > 
> > > Hi Marcelo,
> > >  
> > > > Whats prev_period for, since in practice the period will not change
> > > > between interrupts (OS programs comparator once, or perhaps twice
> > > > during bootup) ?
> > > 
> > > 'prev_period' is needed if a guest o/s changes the comparator period
> > > 'on the fly' (without stopping and restarting the timer).
> > > 
> > > 
> > >              guest o/s changes period
> > >                |
> > >   ti(n-1)      |        ti(n)                          ti(n+1)
> > >     |          v          |                              |
> > >     +---------------------+------------------------------+
> > > 
> > >      <--- prev_period ---> <---------- period ---------->
> > > 
> > > 
> > > The idea is that each timer interrupt represents a certain quantum
> > > of time (the comparator period). If a guest o/s changes the period
> > > between timer interrupt 'n-1' and timer interrupt 'n', I think the
> > > new value should not take effect before timer interrupt 'n'. Timer
> > > interrupt 'n' still represents the old/previous quantum, and timer
> > > interrupt 'n+1' represents the new quantum.
> > > 
> > > Hence, the patch decrements 'ticks_not_accounted' by 'prev_period'
> > > and sets 'prev_period' to 'period' when an interrupt was delivered
> > > to the guest o/s.
> > > 
> > > +            irq_delivered = update_irq(t, 1);
> > > +            if (irq_delivered) {
> > > +                t->ticks_not_accounted -= t->prev_period;
> > > +                t->prev_period = t->period;
> > > +            } else {
> > > 
> > > Most of the time 'prev_period' is equal to 'period'. It should only
> > > be different in the scenario shown above.
> > 
> > OK, makes sense. You should probably reset ticks_not_accounted to zero
> > on HPET initialization (for example, to avoid miscalibration when
> > kexec'ing a new kernel).
> 
> Everybody resetting the machine in anyway is expected to force devices
> to be reinitialized, right ?
> I may be wrong, but I was under the impression that kexec would do this
> as well. In this case, the reset function should be enough.
> 
kexec does not reset a machine. That's the whole point of kexec in
fact.

--
                        Gleb.