[RFC v2 13/15] system/qdev-monitor: Introduce bus-finder interface for compatibility with bus-less plug behavior

[Zhao Liu <zhao1.liu@intel.com>]

Currently, cpu and core is located by topology IDs when plugging.

On a topology tree, each topology device will has a CPU bus. Once cpu
and core specify the bus_type, it's necessary to find accurate buses
for them based on topology IDs (if bus=* is not set in -device).

Therefore, we need a way to use traditional topology IDs for locating
specific bus in the topology tree. This is the bus-finder interface.

With bus-finder, qdev-monitor can locate the bus based on device
properties when "bus=*" is not specified.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
 MAINTAINERS                  |  2 ++
 include/monitor/bus-finder.h | 41 ++++++++++++++++++++++++++++++++
 system/bus-finder.c          | 46 ++++++++++++++++++++++++++++++++++++
 system/meson.build           |  1 +
 system/qdev-monitor.c        | 41 ++++++++++++++++++++++++++++----
 5 files changed, 126 insertions(+), 5 deletions(-)
 create mode 100644 include/monitor/bus-finder.h
 create mode 100644 system/bus-finder.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 03c1a13de074..4608c3c6db8c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3281,12 +3281,14 @@ F: hw/core/qdev*
 F: hw/core/bus.c
 F: hw/core/sysbus.c
 F: include/hw/qdev*
+F: include/monitor/bus-finder.h
 F: include/monitor/qdev.h
 F: include/qom/
 F: qapi/qom.json
 F: qapi/qdev.json
 F: scripts/coccinelle/qom-parent-type.cocci
 F: scripts/qom-cast-macro-clean-cocci-gen.py
+F: system/bus-finder.c
 F: system/qdev-monitor.c
 F: stubs/qdev.c
 F: qom/
diff --git a/include/monitor/bus-finder.h b/include/monitor/bus-finder.h
new file mode 100644
index 000000000000..56f1e4791b66
--- /dev/null
+++ b/include/monitor/bus-finder.h
@@ -0,0 +1,41 @@
+/*
+ * Bus finder interface header
+ *
+ * Copyright (C) 2024 Intel Corporation.
+ *
+ * Author: Zhao Liu <zhao1.liu@intel.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+
+#ifndef BUS_FINDER_H
+#define BUS_FINDER_H
+
+#include "hw/qdev-core.h"
+#include "qom/object.h"
+
+#define TYPE_BUS_FINDER "bus-finder"
+
+typedef struct BusFinderClass BusFinderClass;
+DECLARE_CLASS_CHECKERS(BusFinderClass, BUS_FINDER, TYPE_BUS_FINDER)
+#define BUS_FINDER(obj) INTERFACE_CHECK(BusFinder, (obj), TYPE_BUS_FINDER)
+
+typedef struct BusFinder BusFinder;
+
+/**
+ * BusFinderClass:
+ * @find_bus: Method to find bus.
+ */
+struct BusFinderClass {
+    /* <private> */
+    InterfaceClass parent_class;
+
+    /* <public> */
+    BusState *(*find_bus)(DeviceState *dev);
+};
+
+bool is_bus_finder_type(DeviceClass *dc);
+BusState *bus_finder_select_bus(DeviceState *dev);
+
+#endif /* BUS_FINDER_H */
diff --git a/system/bus-finder.c b/system/bus-finder.c
new file mode 100644
index 000000000000..097291a96bf3
--- /dev/null
+++ b/system/bus-finder.c
@@ -0,0 +1,46 @@
+/*
+ * Bus finder interface
+ *
+ * Copyright (C) 2024 Intel Corporation.
+ *
+ * Author: Zhao Liu <zhao1.liu@intel.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * later.  See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+
+#include "hw/qdev-core.h"
+#include "monitor/bus-finder.h"
+#include "qom/object.h"
+
+bool is_bus_finder_type(DeviceClass *dc)
+{
+    return !!object_class_dynamic_cast(OBJECT_CLASS(dc), TYPE_BUS_FINDER);
+}
+
+BusState *bus_finder_select_bus(DeviceState *dev)
+{
+    BusFinder *bf = BUS_FINDER(dev);
+    BusFinderClass *bfc = BUS_FINDER_GET_CLASS(bf);
+
+    if (bfc->find_bus) {
+        return bfc->find_bus(dev);
+    }
+
+    return NULL;
+}
+
+static const TypeInfo bus_finder_interface_info = {
+    .name          = TYPE_BUS_FINDER,
+    .parent        = TYPE_INTERFACE,
+    .class_size = sizeof(BusFinderClass),
+};
+
+static void bus_finder_register_types(void)
+{
+    type_register_static(&bus_finder_interface_info);
+}
+
+type_init(bus_finder_register_types)
diff --git a/system/meson.build b/system/meson.build
index a296270cb005..090716b81abd 100644
--- a/system/meson.build
+++ b/system/meson.build
@@ -9,6 +9,7 @@ specific_ss.add(when: 'CONFIG_SYSTEM_ONLY', if_true: [files(
 system_ss.add(files(
   'balloon.c',
   'bootdevice.c',
+  'bus-finder.c',
   'cpus.c',
   'cpu-throttle.c',
   'cpu-timers.c',
diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
index 44994ea0e160..457dfd05115e 100644
--- a/system/qdev-monitor.c
+++ b/system/qdev-monitor.c
@@ -19,6 +19,7 @@
 
 #include "qemu/osdep.h"
 #include "hw/sysbus.h"
+#include "monitor/bus-finder.h"
 #include "monitor/hmp.h"
 #include "monitor/monitor.h"
 #include "monitor/qdev.h"
@@ -589,6 +590,16 @@ static BusState *qbus_find(const char *path, Error **errp)
     return bus;
 }
 
+static inline bool qdev_post_find_bus(DeviceClass *dc)
+{
+    return is_bus_finder_type(dc);
+}
+
+static inline BusState *qdev_find_bus_post_device(DeviceState *dev)
+{
+    return bus_finder_select_bus(dev);
+}
+
 /* Takes ownership of @id, will be freed when deleting the device */
 const char *qdev_set_id(DeviceState *dev, char *id, Error **errp)
 {
@@ -630,6 +641,7 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
     char *id;
     DeviceState *dev = NULL;
     BusState *bus = NULL;
+    bool post_bus = false;
 
     driver = qdict_get_try_str(opts, "driver");
     if (!driver) {
@@ -656,11 +668,15 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
             return NULL;
         }
     } else if (dc->bus_type != NULL) {
-        bus = qbus_find_recursive(sysbus_get_default(), NULL, dc->bus_type);
-        if (!bus || qbus_is_full(bus)) {
-            error_setg(errp, "No '%s' bus found for device '%s'",
-                       dc->bus_type, driver);
-            return NULL;
+        if (qdev_post_find_bus(dc)) {
+            post_bus = true;             /* Wait for bus-finder to arbitrate. */
+        } else {
+            bus = qbus_find_recursive(sysbus_get_default(), NULL, dc->bus_type);
+            if (!bus || qbus_is_full(bus)) {
+                error_setg(errp, "No '%s' bus found for device '%s'",
+                           dc->bus_type, driver);
+                return NULL;
+            }
         }
     }
 
@@ -722,6 +738,21 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts,
         goto err_del_dev;
     }
 
+    if (post_bus) {
+        bus = qdev_find_bus_post_device(dev);
+        if (!bus) {
+            error_setg(errp, "No proper '%s' bus found for device '%s'",
+                       dc->bus_type, driver);
+            goto err_del_dev;
+        }
+
+        if (phase_check(PHASE_MACHINE_READY) && !qbus_is_hotpluggable(bus)) {
+            error_setg(errp, "Bus '%s' does not support hotplugging",
+                       bus->name);
+            goto err_del_dev;
+        }
+    }
+
     if (!qdev_realize(dev, bus, errp)) {
         goto err_del_dev;
     }
-- 
2.34.1