powerpc/64s: idle POWER9 can execute stop in virtual mode

The hardware can execute stop in any context, and KVM does not
require real mode because siblings do not share MMU state. This
saves a switch to real-mode when going idle.
Acked-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/idle_book3s.S

@@ -195,7 +195,16 @@ pnv_powersave_common:
 	std	r5,_CCR(r1)
 	std	r1,PACAR1(r13)
 
+BEGIN_FTR_SECTION
+	/*
+	 * POWER9 does not require real mode to stop, and presently does not
+	 * set hwthread_state for KVM (threads don't share MMU context), so
+	 * we can remain in virtual mode for this.
+	 */
+	bctr
+END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 	/*
+	 * POWER8
 	 * Go to real mode to do the nap, as required by the architecture.
 	 * Also, we need to be in real mode before setting hwthread_state,
 	 * because as soon as we do that, another thread can switch