Re: [PATCH 2/3] KVM: PPC: Add SPR emulation exits

[Scott Wood <scottwood@freescale.com>]

On 10/08/2012 04:01:11 PM, Alexander Graf wrote:
> 
> On 08.10.2012, at 22:45, Scott Wood wrote:
> 
> > On 10/07/2012 08:30:06 AM, Alexander Graf wrote:
> >> On 07.10.2012, at 15:26, Avi Kivity wrote:
> >> > The downside of this generic approach is that it prepares  
> suprises down
> >> > the road.  The alternative approach, of adding a new  
> KVM_EXIT_RESET,
> >> > avoids this minefield, but requires ABI changes every time we  
> want to
> >> > emulate something in userspace.  Can you provide a critique of  
> this
> >> > alternate approach?
> >> Yeah, it doesn't scale as well. The SPR read/write give us all  
> information we need to emulate other registers too, like the magical  
> "read this SPR and automatically get the interrupt vector from the  
> MPIC and ack the interrupt along the way" register we have on e500.
> >
> > That's not actually how the register works in hardware (though it  
> may be a reasonable way to emulate it with a userspace mpic).  The  
> interrupt is acknowledged when the core branches to the interrupt  
> vector.  The register itself is just storage that gets filled when  
> that happens.
> 
> Mind to enlighten me again on how exactly this mode gets enabled so  
> that an OS that does not make use of the SPR can still ask the MPIC  
> by hand :)?

GCR[M] is set to 3 for external proxy mode, versus 1 for traditional  
operation.

-Scott