py/vm: Add macros to hook into various points in the VM.

These can be used to insert arbitrary checks, polling, etc into the VM.
They are left general because the VM is a highly tuned loop and it should
be up to a given port how that port wants to modify the VM internals.

One common use would be to insert a polling check, but only done after
a certain number of opcodes were executed, so as not to slow down the VM
too much.  For example:

 #define MICROPY_VM_HOOK_COUNT (30)
 #define MICROPY_VM_HOOK_INIT static uint vm_hook_divisor = MICROPY_VM_HOOK_COUNT
 #define MICROPY_VM_HOOK_POLL if (--vm_hook_divisor == 0) { \
     vm_hook_divisor = MICROPY_VM_HOOK_COUNT;
     extern void vm_hook_function(void);
     vm_hook_function();
 }
 #define MICROPY_VM_HOOK_LOOP MICROPY_VM_HOOK_POLL
 #define MICROPY_VM_HOOK_RETURN MICROPY_VM_HOOK_POLL

py/mpconfig.h

@@ -351,6 +351,22 @@
 /*****************************************************************************/
 /* Python internal features                                                  */
 
+// Hook for the VM at the start of the opcode loop (can contain variable
+// definitions usable by the other hook functions)
+#ifndef MICROPY_VM_HOOK_INIT
+#define MICROPY_VM_HOOK_INIT
+#endif
+
+// Hook for the VM during the opcode loop (but only after jump opcodes)
+#ifndef MICROPY_VM_HOOK_LOOP
+#define MICROPY_VM_HOOK_LOOP
+#endif
+
+// Hook for the VM just before return opcode is finished being interpreted
+#ifndef MICROPY_VM_HOOK_RETURN
+#define MICROPY_VM_HOOK_RETURN
+#endif
+
 // Whether to include the garbage collector
 #ifndef MICROPY_ENABLE_GC
 #define MICROPY_ENABLE_GC (0)

py/vm.c

@@ -179,6 +179,7 @@ outer_dispatch_loop:
             const byte *ip = code_state->ip;
             mp_obj_t *sp = code_state->sp;
             mp_obj_t obj_shared;
+            MICROPY_VM_HOOK_INIT
 
             // If we have exception to inject, now that we finish setting up
             // execution context, raise it. This works as if RAISE_VARARGS
@@ -1069,6 +1070,7 @@ unwind_return:
                     nlr_pop();
                     code_state->sp = sp;
                     assert(exc_sp == exc_stack - 1);
+                    MICROPY_VM_HOOK_RETURN
                     #if MICROPY_STACKLESS
                     if (code_state->prev != NULL) {
                         mp_obj_t res = *sp;
@@ -1252,6 +1254,7 @@ yield:
 #endif
 
 pending_exception_check:
+                MICROPY_VM_HOOK_LOOP
                 if (MP_STATE_VM(mp_pending_exception) != MP_OBJ_NULL) {
                     MARK_EXC_IP_SELECTIVE();
                     mp_obj_t obj = MP_STATE_VM(mp_pending_exception);