migration: Add save_postcopy_prepare() savevm handler

Add a savevm handler for a module to opt-in sending extra sections right
before postcopy starts, and before VM is stopped.

RAM will start to use this new savevm handler in the next patch to do flush
and sync for multifd pages.

Note that we choose to do it before VM stopped because the current only
potential user is not sensitive to VM status, so doing it before VM is
stopped is preferred to enlarge any postcopy downtime.

It is still a bit unfortunate that we need to introduce such a new savevm
handler just for the only use case, however it's so far the cleanest.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250411114534.3370816-4-ppandit@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

include/migration/register.h

@@ -189,6 +189,21 @@ typedef struct SaveVMHandlers {
 
     /* This runs outside the BQL!  */
 
+    /**
+     * @save_postcopy_prepare
+     *
+     * This hook will be invoked on the source side right before switching
+     * to postcopy (before VM stopped).
+     *
+     * @f:      QEMUFile where to send the data
+     * @opaque: Data pointer passed to register_savevm_live()
+     * @errp:   Error** used to report error message
+     *
+     * Returns: true if succeeded, false if error occured.  When false is
+     * returned, @errp must be set.
+     */
+    bool (*save_postcopy_prepare)(QEMUFile *f, void *opaque, Error **errp);
+
     /**
      * @state_pending_estimate
      *

migration/migration.c

@@ -2727,6 +2727,10 @@ static int postcopy_start(MigrationState *ms, Error **errp)
         }
     }
 
+    if (!qemu_savevm_state_postcopy_prepare(ms->to_dst_file, errp)) {
+        return -1;
+    }
+
     trace_postcopy_start();
     bql_lock();
     trace_postcopy_start_set_run();

migration/savevm.c

@@ -1523,6 +1523,39 @@ void qemu_savevm_state_complete_postcopy(QEMUFile *f)
     qemu_fflush(f);
 }
 
+bool qemu_savevm_state_postcopy_prepare(QEMUFile *f, Error **errp)
+{
+    SaveStateEntry *se;
+    bool ret;
+
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (!se->ops || !se->ops->save_postcopy_prepare) {
+            continue;
+        }
+
+        if (se->ops->is_active) {
+            if (!se->ops->is_active(se->opaque)) {
+                continue;
+            }
+        }
+
+        trace_savevm_section_start(se->idstr, se->section_id);
+
+        save_section_header(f, se, QEMU_VM_SECTION_PART);
+        ret = se->ops->save_postcopy_prepare(f, se->opaque, errp);
+        save_section_footer(f, se);
+
+        trace_savevm_section_end(se->idstr, se->section_id, ret);
+
+        if (!ret) {
+            assert(*errp);
+            return false;
+        }
+    }
+
+    return true;
+}
+
 int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
 {
     int64_t start_ts_each, end_ts_each;

migration/savevm.h

@@ -45,6 +45,7 @@ void qemu_savevm_state_pending_exact(uint64_t *must_precopy,
 void qemu_savevm_state_pending_estimate(uint64_t *must_precopy,
                                         uint64_t *can_postcopy);
 int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy);
+bool qemu_savevm_state_postcopy_prepare(QEMUFile *f, Error **errp);
 void qemu_savevm_send_ping(QEMUFile *f, uint32_t value);
 void qemu_savevm_send_open_return_path(QEMUFile *f);
 int qemu_savevm_send_packaged(QEMUFile *f, const uint8_t *buf, size_t len);