Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 0/4] target-ppc: Add FWNMI support in qemu for powerKVM guests

[Paul Mackerras]

On Thu, Sep 03, 2015 at 03:05:21PM +1000, David Gibson wrote:
> On Thu, Sep 03, 2015 at 01:24:21PM +1000, Sam Bobroff wrote:
> > PAPR only says SPRGs 0 to 3 are for software use, but the kernel (see
> > arch/powerpc/include/asm/reg.h) defines SPRG2 as an exception scratch 
> > register
> > so it should be the right one to use here.
> 
> Uh.. no.  If 0..3 are for software (i.e. OS) use, then this needs to
> use a different one, since it's being used as a firmware resource
> here.  Linux might treat SPRG2 as scratch, but another OS would be
> within its rights to use it for something persistent.

PAPR says in requirement R1-14.1.2-3 "To avoid conflict with the
platform’s hypervisor, the OS must be prepared to share use of SPRG2
as the interrupt scratch register whenever an hcall() is made, or a
machine check or reset interrupt is taken."  So, SPRG2 is the one to
use here.

Paul.