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

[Michal Ostrowski <mostrows@xxxxxxxxxxxxxx>]

On Fri, 2006-03-10 at 11:00 -0600, Olof Johansson wrote:
> On Fri, Mar 10, 2006 at 09:30:38AM -0600, Hollis Blanchard wrote:
> > On Friday 10 March 2006 04:07, Tristan Gingold wrote:
> > > 
> > > How xen/ppc delivers interrupts ?
> > 
> > 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 changes 
> > 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.
> > 
> > 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 after 
> > all". Note that in this model, interrupts are delivered by the hardware 
> > directly to the current domain without hypervisor involvement.
> 

Acutally, rhype handles all 0x500 exceptions directly and then simulates
0x500 exceptions to partitions when they are to actually see an
interrupt.  Thus if an interrupt occurs for a partition that is not
currently running, the partition does not see the exception.  PHYP
implements it differently; 0x500 exceptions are always seen by the
currently running partitions even if the interrupt is not for them.

> 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.
> 


PPC970 gives you the choice of where external exceptions are directed
to; hypervisor or partition (e.g. do external interrupts turn on
MSR.HV?).

> Some PPC processors have a feature called "mediated external interrupts",
> 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.
> 
> rHype has some support for this already, but it's unclear which hardware
> has it. If I had to guess I would say at least Cell, since those parts
> of rhype are ripped out in the sources but some of the infrastructure
> is still in there.
> 

rhype interrupt handling  code has been design with mediated interrupts
in mind.  If there are no mediated interrupts certain branches are never
taken and certain things become no-ops.


-- 
Michal Ostrowski <mostrows@xxxxxxxxxxxxxx>


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