armv7m: Escalate exceptions to HardFault if necessary

The v7M exception architecture requires that if a synchronous
exception cannot be taken immediately (because it is disabled
or at too low a priority) then it should be escalated to
HardFault (and the HardFault exception is then taken).
Implement this escalation logic.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
[PMM: extracted from another patch]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

hw/intc/armv7m_nvic.c

@@ -352,6 +352,59 @@ void armv7m_nvic_set_pending(void *opaque, int irq)
 
     vec = &s->vectors[irq];
     trace_nvic_set_pending(irq, vec->enabled, vec->prio);
+
+
+    if (irq >= ARMV7M_EXCP_HARD && irq < ARMV7M_EXCP_PENDSV) {
+        /* If a synchronous exception is pending then it may be
+         * escalated to HardFault if:
+         *  * it is equal or lower priority to current execution
+         *  * it is disabled
+         * (ie we need to take it immediately but we can't do so).
+         * Asynchronous exceptions (and interrupts) simply remain pending.
+         *
+         * For QEMU, we don't have any imprecise (asynchronous) faults,
+         * so we can assume that PREFETCH_ABORT and DATA_ABORT are always
+         * synchronous.
+         * Debug exceptions are awkward because only Debug exceptions
+         * resulting from the BKPT instruction should be escalated,
+         * but we don't currently implement any Debug exceptions other
+         * than those that result from BKPT, so we treat all debug exceptions
+         * as needing escalation.
+         *
+         * This all means we can identify whether to escalate based only on
+         * the exception number and don't (yet) need the caller to explicitly
+         * tell us whether this exception is synchronous or not.
+         */
+        int running = nvic_exec_prio(s);
+        bool escalate = false;
+
+        if (vec->prio >= running) {
+            trace_nvic_escalate_prio(irq, vec->prio, running);
+            escalate = true;
+        } else if (!vec->enabled) {
+            trace_nvic_escalate_disabled(irq);
+            escalate = true;
+        }
+
+        if (escalate) {
+            if (running < 0) {
+                /* We want to escalate to HardFault but we can't take a
+                 * synchronous HardFault at this point either. This is a
+                 * Lockup condition due to a guest bug. We don't model
+                 * Lockup, so report via cpu_abort() instead.
+                 */
+                cpu_abort(&s->cpu->parent_obj,
+                          "Lockup: can't escalate %d to HardFault "
+                          "(current priority %d)\n", irq, running);
+            }
+
+            /* We can do the escalation, so we take HardFault instead */
+            irq = ARMV7M_EXCP_HARD;
+            vec = &s->vectors[irq];
+            s->cpu->env.v7m.hfsr |= R_V7M_HFSR_FORCED_MASK;
+        }
+    }
+
     if (!vec->pending) {
         vec->pending = 1;
         nvic_irq_update(s);

target/arm/helper.c

@@ -6106,8 +6106,6 @@ void arm_v7m_cpu_do_interrupt(CPUState *cs)
 
     /* For exceptions we just mark as pending on the NVIC, and let that
        handle it.  */
-    /* TODO: Need to escalate if the current priority is higher than the
-       one we're raising.  */
     switch (cs->exception_index) {
     case EXCP_UDEF:
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE);