intc/ibex_plic: Ensure we don't loose interrupts

If an interrupt occurs between when we claim and complete an interrupt
we currently drop the interrupt in ibex_plic_irqs_set_pending(). This
somewhat matches hardware that also ignore the interrupt between the
claim and complete process.

In the case of hardware though the physical interrupt line will still
be asserted after we have completed the interrupt. This means we will
still act on the interrupt after the complete process. In QEMU we don't
and instead we drop the interrupt as it is never recorded.

This patch changed the behaviour of the Ibex PLIC so that we save all
interrupts that occur while we are between claiming and completing an
interrupt so that we can act on them after the completition process.

This fixes interrupts being dropped when running Tock on OpenTitain in
QEMU.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: e7bcf98c6925b1e6e7828e7c3f85293a09a65b12.1605136387.git.alistair.francis@wdc.com

hw/intc/ibex_plic.c

@@ -48,6 +48,7 @@ static void ibex_plic_irqs_set_pending(IbexPlicState *s, int irq, bool level)
          * The interrupt has been claimed, but not completed.
          * The pending bit can't be set.
          */
+        s->hidden_pending[pending_num] |= level << (irq % 32);
         return;
     }
 
@@ -176,8 +177,21 @@ static void ibex_plic_write(void *opaque, hwaddr addr,
             s->claim = 0;
         }
         if (s->claimed[value / 32] & 1 << (value % 32)) {
+            int pending_num = value / 32;
+
             /* This value was already claimed, clear it. */
-            s->claimed[value / 32] &= ~(1 << (value % 32));
+            s->claimed[pending_num] &= ~(1 << (value % 32));
+
+            if (s->hidden_pending[pending_num] & (1 << (value % 32))) {
+                /*
+                 * If the bit in hidden_pending is set then that means we
+                 * received an interrupt between claiming and completing
+                 * the interrupt that hasn't since been de-asserted.
+                 * On hardware this would trigger an interrupt, so let's
+                 * trigger one here as well.
+                 */
+                s->pending[pending_num] |= 1 << (value % 32);
+            }
         }
     }
 
@@ -239,6 +253,7 @@ static void ibex_plic_realize(DeviceState *dev, Error **errp)
     int i;
 
     s->pending = g_new0(uint32_t, s->pending_num);
+    s->hidden_pending = g_new0(uint32_t, s->pending_num);
     s->claimed = g_new0(uint32_t, s->pending_num);
     s->source = g_new0(uint32_t, s->source_num);
     s->priority = g_new0(uint32_t, s->priority_num);

include/hw/intc/ibex_plic.h

@@ -33,6 +33,7 @@ struct IbexPlicState {
     MemoryRegion mmio;
 
     uint32_t *pending;
+    uint32_t *hidden_pending;
     uint32_t *claimed;
     uint32_t *source;
     uint32_t *priority;