nvic: Don't apply group priority mask to negative priorities

In several places we were unconditionally applying the
nvic_gprio_mask() to a priority value. This is incorrect
if the priority is one of the fixed negative priority
values (for NMI and HardFault), so don't do it.

This bug would have caused both NMI and HardFault to be
considered as the same priority and so NMI wouldn't
correctly preempt HardFault.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 1505137930-13255-5-git-send-email-peter.maydell@linaro.org

hw/intc/armv7m_nvic.c

@@ -152,8 +152,12 @@ static void nvic_recompute_state(NVICState *s)
         }
     }
 
+    if (active_prio > 0) {
+        active_prio &= nvic_gprio_mask(s);
+    }
+
     s->vectpending = pend_irq;
-    s->exception_prio = active_prio & nvic_gprio_mask(s);
+    s->exception_prio = active_prio;
 
     trace_nvic_recompute_state(s->vectpending, s->exception_prio);
 }
@@ -329,7 +333,10 @@ void armv7m_nvic_acknowledge_irq(void *opaque)
     assert(vec->enabled);
     assert(vec->pending);
 
-    pendgroupprio = vec->prio & nvic_gprio_mask(s);
+    pendgroupprio = vec->prio;
+    if (pendgroupprio > 0) {
+        pendgroupprio &= nvic_gprio_mask(s);
+    }
     assert(pendgroupprio < running);
 
     trace_nvic_acknowledge_irq(pending, vec->prio);