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Date: Wed, 4 May 2011 04:06:59 -0400 (EDT)
From: Ulrich Obergfell <uobergfe@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 5/5] hpet 'driftfix': add code in
 hpet_timer() to compensate delayed callbacks and coalesced interrupts
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To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: aliguori@us.ibm.com, kvm@vger.kernel.org, jan kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, gcosta@redhat.com, avi@redhat.com


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