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.
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