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RE: [Xen-devel] Re: [XenPPC] Re: IRQs delivery.

>From: Olof Johansson
>Sent: 2006年3月11日 1:00
>> Ignoring hypervisors for a moment, the PPC model for external
>interrupts is
>> this: the hardware resets the PC to a fixed entry point (0x500) and
>> the MSR to put the processor into real (untranslated) mode. (The old
>PC and
>> MSR are saved into a pair of supervisor-only registers, SRR0 and
>SRR1.) The
>> exception handler is then responsible for querying the interrupt
>> controller(s) to get the interrupt vector, and then call the appropriate
>> driver.

Exact same process on most archs, except switching into real mode. 
For IA64, PC->IP, MSR->PSR(processor status register), SRR0->IIP, SRR1->IPSR, 
0x500->IVA+0x3000(Interruption Vector Address). A typical 
flow here. :-)

>> The IBM Research hypervisor (rhype) virtualized the PIC, so that
>instead of
>> querying hardware, the kernel makes an hcall to find the incoming
>vector. If
>> the interrupt is not in fact for this domain, the hypervisor queues the
>> interrupt for later and tells the current domain "nothing was pending
>> all". Note that in this model, interrupts are delivered by the hardware
>> directly to the current domain without hypervisor involvement.
>Not that you have a choice on PPC970, but there's a drawback to this: If
>you let all interrupts be delivered directly to the domain, then it can
>hold the PIC "hostage" by disabling the delivery (keeping MSR_EE off),
>causing interrupts to other domains to be delayed.

Seems issue for driver domain, but good performance for current model.

>Some PPC processors have a feature called "mediated external
>where they will be delivered to the hypervisor instead, and if the domain
>can't service it then (MSR_EE off), the HV can request to be notified
>when the domain can take it. The extra code path for re-delivering to a
>domain can be made short.

So this "mediated external interrupt" is only a new hardware feature for 
virtualization? Can native OS take use of it?

>I'm just mentioning this in case the IA64 guys have something like
>mediated interrupts as an option, they might want to go with that for
>the fairness/latency reasons.

On IA64, IVA always points to the base address of table containing 
exception handlers which is a system resource and owned by Xen 
hypervisor. Once interrupt happens, it's always xen hypervisor to be
 notified first. So you may think IA64 is always "mediated external interrupt" 
by your term.


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