powerpc/64s: fix idle wakeup potential to clobber registers

[Michael Ellerman]

On Fri, 2017-03-17 at 05:13:20 UTC, Nicholas Piggin wrote:
> We concluded there may be a window where the idle wakeup code could
> get to pnv_wakeup_tb_loss (which clobbers non-volatile GPRs), but the
> hardware may set SRR1[46:47] to 01b (no state loss) which would
> result in the wakeup code failing to restore non-volatile GPRs.
> 
> I was not able to trigger this condition with trivial tests on
> real hardware or simulator, but the ISA (at least 2.07) seems to
> allow for it, and Gautham says that it can happen if there is an
> exception pending when the sleep/winkle instruction is executed.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> Acked-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/6d98ce0be541d4a3cfbb52cd75072c

cheers