Subject: Re: [PATCH] powerpc/booke64: Configurable lazy interrupt disabling

From: Scott Wood <[email-masked]>
Date: Mon, 23 Jan 2012 13:31:35 -0600

On 01/20/2012 05:02 PM, Benjamin Herrenschmidt wrote:
> 
>> With Topaz, interrupts go directly to guests and we don't want to require a
>> trap/hcall to do an IACK, as that adds potentially thousands of cycles of
>> latency to every interrupt.
>>
>> As you know, with external proxy interrupts are acknowledged by the
>> hardware and it becomes problematic to replay the interrupt in
>> the context of lazy EE when interrupts are re-enabled.   The interrupt
>> will not fire again when you enable EE.
> 
> So you have a broken HW and broken HV design to start with ... :-)

That "breakage" significantly reduces the overhead of handling
interrupts in a guest, since we don't need to involve the hypervisor (at
least on the entry side -- there's a hack to avoid it on EOI as well,
though that involves exposing some MPIC registers directly; users can
decide whether it's worth enabling for their use case).

-Scott