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).
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