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.
Regards,
Uli
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