cpu/core: Fix "help" of CPU core device types

Calling qdev_get_machine() from a QOM instance_init function is
fragile because we can't be sure the machine object actually
exists. And this happens to break when passing ",help" on the
command line to get the list of properties for a CPU core
device types :

$ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help
qemu-system-ppc64: ../../hw/core/machine.c:1290:
 qdev_get_machine: Assertion `machine != NULL' failed.
Aborted (core dumped)

This used to work before QEMU 5.0, but commit 3df261b6676b
unwillingly introduced a subtle regression : the above command
line needs to create an instance but the instance_init function
of the base class calls qdev_get_machine() before
qemu_create_machine() has been called, which is a programming bug.

Use current_machine instead. It is okay to skip the setting of
nr_thread in this case since only its type is displayed.

Fixes: 3df261b6676b ("softmmu/vl.c: Handle '-cpu help' and '-device help' before 'no default machine'")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Cc: peter.maydell@linaro.org
Message-Id: <20210409160339.500167-3-groug@kaod.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

hw/cpu/core.c

@@ -66,10 +66,16 @@ static void core_prop_set_nr_threads(Object *obj, Visitor *v, const char *name,
 
 static void cpu_core_instance_init(Object *obj)
 {
-    MachineState *ms = MACHINE(qdev_get_machine());
     CPUCore *core = CPU_CORE(obj);
 
-    core->nr_threads = ms->smp.threads;
+    /*
+     * Only '-device something-cpu-core,help' can get us there before
+     * the machine has been created. We don't care to set nr_threads
+     * in this case since it isn't used afterwards.
+     */
+    if (current_machine) {
+        core->nr_threads = current_machine->smp.threads;
+    }
 }
 
 static void cpu_core_class_init(ObjectClass *oc, void *data)