powerpc/64: Fix __check_irq_replay missing decrementer interrupt

If the decrementer wraps again and de-asserts the decrementer
exception while hard-disabled, __check_irq_replay() has a test to
notice the wrap when interrupts are re-enabled.

The decrementer check must be done when clearing the PACA_IRQ_HARD_DIS
flag, not when the PACA_IRQ_DEC flag is tested. Previously this worked
because the decrementer interrupt was always the first one checked
after clearing the hard disable flag, but HMI check was moved ahead of
that, which introduced this bug.

This can cause a missed decrementer interrupt if we soft-disable
interrupts then take an HMI which is recorded in irq_happened, then
hard-disable interrupts for > 4s to wrap the decrementer.

Fixes: e0e0d6b7 ("powerpc/64: Replay hypervisor maintenance interrupt first")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/irq.c

@@ -145,6 +145,19 @@ notrace unsigned int __check_irq_replay(void)
 
 	/* Clear bit 0 which we wouldn't clear otherwise */
 	local_paca->irq_happened &= ~PACA_IRQ_HARD_DIS;
+	if (happened & PACA_IRQ_HARD_DIS) {
+		/*
+		 * We may have missed a decrementer interrupt if hard disabled.
+		 * Check the decrementer register in case we had a rollover
+		 * while hard disabled.
+		 */
+		if (!(happened & PACA_IRQ_DEC)) {
+			if (decrementer_check_overflow()) {
+				local_paca->irq_happened |= PACA_IRQ_DEC;
+				happened |= PACA_IRQ_DEC;
+			}
+		}
+	}
 
 	/*
 	 * Force the delivery of pending soft-disabled interrupts on PS3.
@@ -170,7 +183,7 @@ notrace unsigned int __check_irq_replay(void)
 	 * in case we also had a rollover while hard disabled
 	 */
 	local_paca->irq_happened &= ~PACA_IRQ_DEC;
-	if ((happened & PACA_IRQ_DEC) || decrementer_check_overflow())
+	if (happened & PACA_IRQ_DEC)
 		return 0x900;
 
 	/* Finally check if an external interrupt happened */