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

Scott Wood
2012-10-08T20:45:55Z

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.

-Scott