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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