Kumar Gala writes:

quoted

Wouldn't it be better and safer to have the exception prolog for
critical interrupts save SRR0/1 in the stack frame, and have the
prolog for machine checks save SRR0/1 and CSRR0/1 likewise?

If we do this I guess we can use SRR0/1 regardless of which level we  
came from.

That's the idea.  I actually thought the booke exception prologs
already did that, but it seems I'm wrong.

What sort of things do you expect critical and machine-check handlers
to be needing to do?

Paul.