Set SVC one lower in priority

Setting the SVC as one lower in priority allows us to combine MPU
recovery in one place, and not have to recover from a hard fault. We
also use this commit as an opportunity to make the priorities of all
system interrupts explicit.

Author: Patater

api/inc/unvic_exports.h

@@ -21,7 +21,7 @@
 
 /* this value refers to the minimum allowable priority in the physical NVIC
  * module, but not in the virtualised one (vIRQ) */
-#define __UVISOR_NVIC_MIN_PRIORITY ((uint32_t) 1)
+#define __UVISOR_NVIC_MIN_PRIORITY ((uint32_t) 2)
 
 /* this is the maximum priority allowed for the vIRQ module */
 /* users of uVisor APIs can use this to determine the maximum level of

core/system/src/unvic.c

@@ -583,6 +583,21 @@ void unvic_init(void)
      * levels. */
     assert(__UVISOR_NVIC_MIN_PRIORITY < UVISOR_VIRQ_MAX_PRIORITY);
 
+    /* Set the priority of each exception. SVC is lower priority than
+     * MemManage, BusFault, and UsageFault, so that we can recover from
+     * stacking MemManage faults more simply. */
+    static const uint32_t priority_0 = __UVISOR_NVIC_MIN_PRIORITY - 2;
+    static const uint32_t priority_1 = __UVISOR_NVIC_MIN_PRIORITY - 1;
+    assert(priority_0 < __UVISOR_NVIC_MIN_PRIORITY);
+    assert(priority_1 < __UVISOR_NVIC_MIN_PRIORITY);
+    NVIC_SetPriority(MemoryManagement_IRQn, priority_0);
+    NVIC_SetPriority(BusFault_IRQn, priority_0);
+    NVIC_SetPriority(UsageFault_IRQn, priority_0);
+    NVIC_SetPriority(SVCall_IRQn, priority_1);
+    NVIC_SetPriority(DebugMonitor_IRQn, __UVISOR_NVIC_MIN_PRIORITY);
+    NVIC_SetPriority(PendSV_IRQn, UVISOR_VIRQ_MAX_PRIORITY);
+    NVIC_SetPriority(SysTick_IRQn, UVISOR_VIRQ_MAX_PRIORITY);
+
     /* By setting the priority group to 0 we make sure that all priority levels
      * are available for pre-emption and that interrupts with the same priority
      * level occurring at the same time are served in the default way, that is,