hw/intc: ibex_plic: Honour source priorities

This patch follows what commit aa4d30f6618dc "riscv: plic: Honour source
priorities" does and ensures that the highest priority interrupt will be
serviced first.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <a697ca8a31eff8eb18a88e09a28206063cf85d48.1595655188.git.alistair.francis@wdc.com>

hw/intc/ibex_plic.c

@@ -57,6 +57,8 @@ static void ibex_plic_irqs_set_pending(IbexPlicState *s, int irq, bool level)
 static bool ibex_plic_irqs_pending(IbexPlicState *s, uint32_t context)
 {
     int i;
+    uint32_t max_irq = 0;
+    uint32_t max_prio = s->threshold;
 
     for (i = 0; i < s->pending_num; i++) {
         uint32_t irq_num = ctz64(s->pending[i]) + (i * 32);
@@ -66,14 +68,17 @@ static bool ibex_plic_irqs_pending(IbexPlicState *s, uint32_t context)
             continue;
         }
 
-        if (s->priority[irq_num] > s->threshold) {
-            if (!s->claim) {
-                s->claim = irq_num;
-            }
-            return true;
+        if (s->priority[irq_num] > max_prio) {
+            max_irq = irq_num;
+            max_prio = s->priority[irq_num];
         }
     }
 
+    if (max_irq) {
+        s->claim = max_irq;
+        return true;
+    }
+
     return false;
 }