Re: RFC: PIC: enhance IPI avoidance

Avi Kivity
2008-07-13T16:12:26Z

Avi Kivity wrote:
> Marcelo Tosatti wrote:
>> The PIC code makes little effort to avoid kvm_vcpu_kick(), resulting in
>> unnecessary guest exits in some conditions.
>>
>> For example, if the timer interrupt is routed through the IOAPIC, IRR
>> for IRQ 0 will get set but not cleared, since the APIC is handling the
>> acks.
>>
>> This means that everytime an interrupt < 16 is triggered, the priority
>> logic will find IRQ0 pending and send an IPI to vcpu0 (in case IRQ0 is
>> not masked, which is Linux's case).
>>
>> Introduce a new variable isr_ack to represent the IRQ's for which the
>> guest has been signalled / cleared the ISR. Use it to avoid more than
>> one IPI per trigger-ack cycle, in addition to the avoidance when ISR is
>> set in get_priority().
>>
>
> Looks good.

... but perhaps it can be done less intrusively. Presumably the PIC is
masked through the local apic LVT? If so we can check that instead.

So when the lapic detects that extint delivery is disabled, it can call
kvm_pic_disable_delivery(), or something, to reduce pic chatter.

--
error compiling committee.c: too many arguments to function