hw/core: Introduce MachineClass::get_default_cpu_type() helper

MachineClass::get_default_cpu_type() runs once the machine is
created, being able to evaluate runtime checks; it returns the
machine default CPU type.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20251020221508.67413-7-philmd@linaro.org>

hw/core/machine.c

@@ -1556,6 +1556,16 @@ const char *machine_class_default_cpu_type(MachineClass *mc)
     return mc->default_cpu_type;
 }
 
+const char *machine_default_cpu_type(const MachineState *ms)
+{
+    MachineClass *mc = MACHINE_GET_CLASS(ms);
+
+    if (mc->get_default_cpu_type) {
+        return mc->get_default_cpu_type(ms);
+    }
+    return machine_class_default_cpu_type(mc);
+}
+
 static bool is_cpu_type_supported(const MachineState *machine, Error **errp)
 {
     MachineClass *mc = MACHINE_GET_CLASS(machine);

include/hw/boards.h

@@ -25,6 +25,11 @@ OBJECT_DECLARE_TYPE(MachineState, MachineClass, MACHINE)
 
 extern MachineState *current_machine;
 
+/**
+ * machine_default_cpu_type: Return the machine default CPU type.
+ * @ms: Machine state
+ */
+const char *machine_default_cpu_type(const MachineState *ms);
 /**
  * machine_class_default_cpu_type: Return the machine default CPU type.
  * @mc: Machine class
@@ -303,6 +308,7 @@ struct MachineClass {
     int numa_mem_align_shift;
     const char * const *valid_cpu_types;
     GPtrArray *(*get_valid_cpu_types)(const MachineState *ms);
+    const char *(*get_default_cpu_type)(const MachineState *ms);
     strList *allowed_dynamic_sysbus_devices;
     bool auto_enable_numa_with_memhp;
     bool auto_enable_numa_with_memdev;

system/vl.c

@@ -3817,7 +3817,7 @@ void qemu_init(int argc, char **argv)
     migration_object_init();
 
     /* parse features once if machine provides default cpu_type */
-    current_machine->cpu_type = machine_class_default_cpu_type(machine_class);
+    current_machine->cpu_type = machine_default_cpu_type(current_machine);
     if (cpu_option) {
         current_machine->cpu_type = parse_cpu_option(cpu_option);
     }