powerpc/xive: Discard ESB load value when interrupt is invalid

A load on an ESB page returning all 1's means that the underlying
device has invalidated the access to the PQ state of the interrupt
through mmio. It may happen, for example when querying a PHB interrupt
while the PHB is in an error state.

In that case, we should consider the interrupt to be invalid when
checking its state in the irq_get_irqchip_state() handler.

Fixes: da15c03b ("powerpc/xive: Implement get_irqchip_state method for XIVE to fix shutdown race")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
[clg: wrote a commit log, introduced XIVE_ESB_INVALID ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200113130118.27969-1-clg@kaod.org

arch/powerpc/include/asm/xive-regs.h

@@ -39,6 +39,7 @@
 
 #define XIVE_ESB_VAL_P		0x2
 #define XIVE_ESB_VAL_Q		0x1
+#define XIVE_ESB_INVALID	0xFF
 
 /*
  * Thread Management (aka "TM") registers

arch/powerpc/sysdev/xive/common.c

@@ -972,12 +972,21 @@ static int xive_get_irqchip_state(struct irq_data *data,
 				  enum irqchip_irq_state which, bool *state)
 {
 	struct xive_irq_data *xd = irq_data_get_irq_handler_data(data);
+	u8 pq;
 
 	switch (which) {
 	case IRQCHIP_STATE_ACTIVE:
-		*state = !xd->stale_p &&
-			 (xd->saved_p ||
-			  !!(xive_esb_read(xd, XIVE_ESB_GET) & XIVE_ESB_VAL_P));
+		pq = xive_esb_read(xd, XIVE_ESB_GET);
+
+		/*
+		 * The esb value being all 1's means we couldn't get
+		 * the PQ state of the interrupt through mmio. It may
+		 * happen, for example when querying a PHB interrupt
+		 * while the PHB is in an error state. We consider the
+		 * interrupt to be inactive in that case.
+		 */
+		*state = (pq != XIVE_ESB_INVALID) && !xd->stale_p &&
+			(xd->saved_p || !!(pq & XIVE_ESB_VAL_P));
 		return 0;
 	default:
 		return -EINVAL;