powerpc/64s/exception: Avoid touching the stack in hdecrementer

The hdec interrupt handler is reported to sometimes fire in Linux if
KVM leaves it pending after a guest exists. This is harmless, so there
is a no-op handler for it.

The interrupt handler currently uses the regular kernel stack. Change
this to avoid touching the stack entirely.

This should be the last place where the regular Linux stack can be
accessed with asynchronous interrupts (including PMI) soft-masked.
It might be possible to take advantage of this invariant, e.g., to
context switch the kernel stack SLB entry without clearing MSR[EE].

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-17-npiggin@gmail.com

arch/powerpc/kernel/time.c

@@ -663,15 +663,6 @@ void timer_broadcast_interrupt(void)
 }
 #endif
 
-/*
- * Hypervisor decrementer interrupts shouldn't occur but are sometimes
- * left pending on exit from a KVM guest.  We don't need to do anything
- * to clear them, as they are edge-triggered.
- */
-void hdec_interrupt(struct pt_regs *regs)
-{
-}
-
 #ifdef CONFIG_SUSPEND
 static void generic_suspend_disable_irqs(void)
 {