sysemu: Add prepare callback to struct VMChangeStateEntry

Add prepare callback to struct VMChangeStateEntry.

The prepare callback is optional and can be set by the new function
qemu_add_vm_change_state_handler_prio_full() that allows setting this
callback in addition to the main callback.

The prepare callbacks and main callbacks are called in two separate
phases: First all prepare callbacks are called and only then all main
callbacks are called.

The purpose of the new prepare callback is to allow all devices to run a
preliminary task before calling the devices' main callbacks.

This will facilitate adding P2P support for VFIO migration where all
VFIO devices need to be put in an intermediate P2P quiescent state
before being stopped or started by the main callback.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

include/sysemu/runstate.h

@@ -16,6 +16,10 @@ VMChangeStateEntry *qemu_add_vm_change_state_handler(VMChangeStateHandler *cb,
                                                      void *opaque);
 VMChangeStateEntry *qemu_add_vm_change_state_handler_prio(
         VMChangeStateHandler *cb, void *opaque, int priority);
+VMChangeStateEntry *
+qemu_add_vm_change_state_handler_prio_full(VMChangeStateHandler *cb,
+                                           VMChangeStateHandler *prepare_cb,
+                                           void *opaque, int priority);
 VMChangeStateEntry *qdev_add_vm_change_state_handler(DeviceState *dev,
                                                      VMChangeStateHandler *cb,
                                                      void *opaque);

softmmu/runstate.c

@@ -271,6 +271,7 @@ void qemu_system_vmstop_request(RunState state)
 }
 struct VMChangeStateEntry {
     VMChangeStateHandler *cb;
+    VMChangeStateHandler *prepare_cb;
     void *opaque;
     QTAILQ_ENTRY(VMChangeStateEntry) entries;
     int priority;
@@ -293,12 +294,39 @@ static QTAILQ_HEAD(, VMChangeStateEntry) vm_change_state_head =
  */
 VMChangeStateEntry *qemu_add_vm_change_state_handler_prio(
         VMChangeStateHandler *cb, void *opaque, int priority)
+{
+    return qemu_add_vm_change_state_handler_prio_full(cb, NULL, opaque,
+                                                      priority);
+}
+
+/**
+ * qemu_add_vm_change_state_handler_prio_full:
+ * @cb: the main callback to invoke
+ * @prepare_cb: a callback to invoke before the main callback
+ * @opaque: user data passed to the callbacks
+ * @priority: low priorities execute first when the vm runs and the reverse is
+ *            true when the vm stops
+ *
+ * Register a main callback function and an optional prepare callback function
+ * that are invoked when the vm starts or stops running. The main callback and
+ * the prepare callback are called in two separate phases: First all prepare
+ * callbacks are called and only then all main callbacks are called. As its
+ * name suggests, the prepare callback can be used to do some preparatory work
+ * before invoking the main callback.
+ *
+ * Returns: an entry to be freed using qemu_del_vm_change_state_handler()
+ */
+VMChangeStateEntry *
+qemu_add_vm_change_state_handler_prio_full(VMChangeStateHandler *cb,
+                                           VMChangeStateHandler *prepare_cb,
+                                           void *opaque, int priority)
 {
     VMChangeStateEntry *e;
     VMChangeStateEntry *other;
 
     e = g_malloc0(sizeof(*e));
     e->cb = cb;
+    e->prepare_cb = prepare_cb;
     e->opaque = opaque;
     e->priority = priority;
 
@@ -333,10 +361,22 @@ void vm_state_notify(bool running, RunState state)
     trace_vm_state_notify(running, state, RunState_str(state));
 
     if (running) {
+        QTAILQ_FOREACH_SAFE(e, &vm_change_state_head, entries, next) {
+            if (e->prepare_cb) {
+                e->prepare_cb(e->opaque, running, state);
+            }
+        }
+
         QTAILQ_FOREACH_SAFE(e, &vm_change_state_head, entries, next) {
             e->cb(e->opaque, running, state);
         }
     } else {
+        QTAILQ_FOREACH_REVERSE_SAFE(e, &vm_change_state_head, entries, next) {
+            if (e->prepare_cb) {
+                e->prepare_cb(e->opaque, running, state);
+            }
+        }
+
         QTAILQ_FOREACH_REVERSE_SAFE(e, &vm_change_state_head, entries, next) {
             e->cb(e->opaque, running, state);
         }