armv7m: Fix condition check for taking exceptions

The M profile condition for when we can take a pending exception or
interrupt is not the same as that for A/R profile.  The code
originally copied from the A/R profile version of the
cpu_exec_interrupt function only worked by chance for the
very simple case of exceptions being masked by PRIMASK.
Replace it with a call to a function in the NVIC code that
correctly compares the priority of the pending exception
against the current execution priority of the CPU.

[Michael Davidsaver's patchset had a patch to do something
similar but the implementation ended up being a rewrite.]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

hw/intc/armv7m_nvic.c

@@ -286,6 +286,13 @@ static inline int nvic_exec_prio(NVICState *s)
     return MIN(running, s->exception_prio);
 }
 
+bool armv7m_nvic_can_take_pending_exception(void *opaque)
+{
+    NVICState *s = opaque;
+
+    return nvic_exec_prio(s) > nvic_pending_prio(s);
+}
+
 /* caller must call nvic_irq_update() after this */
 static void set_prio(NVICState *s, unsigned irq, uint8_t prio)
 {

target/arm/cpu.c

@@ -338,13 +338,6 @@ static bool arm_v7m_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
     CPUARMState *env = &cpu->env;
     bool ret = false;
 
-
-    if (interrupt_request & CPU_INTERRUPT_FIQ
-        && !(env->daif & PSTATE_F)) {
-        cs->exception_index = EXCP_FIQ;
-        cc->do_interrupt(cs);
-        ret = true;
-    }
     /* ARMv7-M interrupt return works by loading a magic value
      * into the PC.  On real hardware the load causes the
      * return to occur.  The qemu implementation performs the
@@ -354,9 +347,16 @@ static bool arm_v7m_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
      * the stack if an interrupt occurred at the wrong time.
      * We avoid this by disabling interrupts when
      * pc contains a magic address.
+     *
+     * ARMv7-M interrupt masking works differently than -A or -R.
+     * There is no FIQ/IRQ distinction. Instead of I and F bits
+     * masking FIQ and IRQ interrupts, an exception is taken only
+     * if it is higher priority than the current execution priority
+     * (which depends on state like BASEPRI, FAULTMASK and the
+     * currently active exception).
      */
     if (interrupt_request & CPU_INTERRUPT_HARD
-        && !(env->daif & PSTATE_I)
+        && (armv7m_nvic_can_take_pending_exception(env->nvic))
         && (env->regs[15] < 0xfffffff0)) {
         cs->exception_index = EXCP_IRQ;
         cc->do_interrupt(cs);

target/arm/cpu.h

@@ -1356,6 +1356,14 @@ uint32_t arm_phys_excp_target_el(CPUState *cs, uint32_t excp_idx,
                                  uint32_t cur_el, bool secure);
 
 /* Interface between CPU and Interrupt controller.  */
+#ifndef CONFIG_USER_ONLY
+bool armv7m_nvic_can_take_pending_exception(void *opaque);
+#else
+static inline bool armv7m_nvic_can_take_pending_exception(void *opaque)
+{
+    return true;
+}
+#endif
 void armv7m_nvic_set_pending(void *opaque, int irq);
 int armv7m_nvic_acknowledge_irq(void *opaque);
 void armv7m_nvic_complete_irq(void *opaque, int irq);