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

[Marcelo Tosatti]

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).