powerpc/64s: Fix doorbell wakeup msgclr optimisation

Commit 3282a3da25bd ("powerpc/64: Implement soft interrupt replay in C")
broke the doorbell wakeup optimisation introduced by commit a9af97aa0a12
("powerpc/64s: msgclr when handling doorbell exceptions from system
reset").

This patch restores the msgclr, in C code. It's now done in the system
reset wakeup path rather than doorbell interrupt replay where it used
to be, because it is always the right thing to do in the wakeup case,
but it may be rarely of use in other interrupt replay situations in
which case it's wasted work - we would have to run measurements to see
if that was a worthwhile optimisation, and I suspect it would not be.

The results are similar to those in the original commit, test on POWER8
of context_switch selftests benchmark with polling idle disabled (e.g.,
always nap, giving cross-CPU IPIs) gives the following results:

                                  broken           patched
  Different threads, same core:   317k/s           375k/s    +18.7%
  Different cores:                280k/s           282k/s     +1.0%

Fixes: 3282a3da25bd ("powerpc/64: Implement soft interrupt replay in C")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200402121212.1118218-1-npiggin@gmail.com

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index a25ed47087ee..1f1169856dc8 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -527,6 +527,19 @@ void irq_set_pending_from_srr1(unsigned long srr1)
 		return;
 	}
 
+	if (reason == PACA_IRQ_DBELL) {
+		/*
+		 * When doorbell triggers a system reset wakeup, the message
+		 * is not cleared, so if the doorbell interrupt is replayed
+		 * and the IPI handled, the doorbell interrupt would still
+		 * fire when EE is enabled.
+		 *
+		 * To avoid taking the superfluous doorbell interrupt,
+		 * execute a msgclr here before the interrupt is replayed.
+		 */
+		ppc_msgclr(PPC_DBELL_MSGTYPE);
+	}
+
 	/*
 	 * The 0 index (SRR1[42:45]=b0000) must always evaluate to 0,
 	 * so this can be called unconditionally with the SRR1 wake