Subject: Re: [PATCH 10/13] kvm/powerpc: Add support for Book3S processors in hypervisor mode

From: Segher Boessenkool <[email-masked]>
Date: Fri, 27 May 2011 22:59:41 +0200

>>> I do the check there because I was having problems where, if the HDEC
>>> goes negative before we do the partition switch, we would 
>>> occasionally
>>> not get the HDEC interrupt at all until the next time HDEC went
>>> negative, ~ 8.4 seconds later.
>>
>> Yikes - so HDEC is edge and doesn't even keep the interrupt line up?
>> That sounds like a serious hardware limitation. What if you only use
>> HDEC and it triggers while interrupts are off in a critical section?
>> Is the hardware really that broken?
>
> If HDEC expires when interrupts are off, the HDEC interrupt stays
> pending until interrupts get re-enabled.  I'm not sure exactly what
> the conditions are that cause an HDEC interrupt to get lost, but they
> seem to involve at least a partition switch.

On some CPUs, if the top bit of the decrementer is 0 again when you 
re-enable
the interrupt, the interrupt is lost (so it is actually 
level-triggered).
The arch books talk a bit about this AFAIR.

Segher