[PATCH] powerpc/64s: Fix hypercall entry clobbering r12 input

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> A previous optimisation incorrectly assumed the PAPR hcall does
> not use r12, and clobbers it upon entry. In fact it is used as
> an input. This can result in KVM guests crashing (observed with
> PR KVM).
>
> Instead of using r12 to save r13, tihs patch saves r13 in ctr.
> This is more costly, but not as slow as using the SPRG.
>
> Fixes: acd7d8cef0153 ("powerpc/64s: Optimize hypercall/syscall entry")
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>
> ---
> One brown paper bag please.

We're all out, Ben, Mikey and I used them all up :)

cheers