[PATCH RFC 16/26] migration: Add save_live_complete_precopy_async{, wait} handlers

["Maciej S. Szmigiero" <mail@maciej.szmigiero.name>]

From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

These SaveVMHandlers allow device to provide its own asynchronous
transmission of the remaining data at the end of a precopy phase.

The save_live_complete_precopy_async handler is supposed to start such
transmission (for example, by launching appropriate threads) while the
save_live_complete_precopy_async_wait handler is supposed to wait until
such transfer has finished (for example, until the sending threads
have exited).

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
---
 include/migration/register.h | 31 +++++++++++++++++++++++++++++++
 migration/savevm.c           | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/include/migration/register.h b/include/migration/register.h
index d7b70a8be68c..9d36e35bd612 100644
--- a/include/migration/register.h
+++ b/include/migration/register.h
@@ -102,6 +102,37 @@ typedef struct SaveVMHandlers {
      */
     int (*save_live_complete_precopy)(QEMUFile *f, void *opaque);
 
+    /**
+     * @save_live_complete_precopy_async
+     *
+     * Arranges for handler-specific asynchronous transmission of the
+     * remaining data at the end of a precopy phase. When postcopy is
+     * enabled, devices that support postcopy will skip this step.
+     *
+     * @f: QEMUFile where the handler can synchronously send data before returning
+     * @idstr: this device section idstr
+     * @instance_id: this device section instance_id
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
+    int (*save_live_complete_precopy_async)(QEMUFile *f,
+                                            char *idstr, uint32_t instance_id,
+                                            void *opaque);
+    /**
+     * @save_live_complete_precopy_async_wait
+     *
+     * Waits for the asynchronous transmission started by the of the
+     * @save_live_complete_precopy_async handler to complete.
+     * When postcopy is enabled, devices that support postcopy will skip this step.
+     *
+     * @f: QEMUFile where the handler can synchronously send data before returning
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
+    int (*save_live_complete_precopy_async_wait)(QEMUFile *f, void *opaque);
+
     /* This runs both outside and inside the BQL.  */
 
     /**
diff --git a/migration/savevm.c b/migration/savevm.c
index 388d7af7cdd8..fa35504678bf 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1497,6 +1497,27 @@ int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
     SaveStateEntry *se;
     int ret;
 
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (!se->ops || (in_postcopy && se->ops->has_postcopy &&
+             se->ops->has_postcopy(se->opaque)) ||
+            !se->ops->save_live_complete_precopy_async) {
+            continue;
+        }
+
+        save_section_header(f, se, QEMU_VM_SECTION_END);
+
+        ret = se->ops->save_live_complete_precopy_async(f,
+                                                        se->idstr, se->instance_id,
+                                                        se->opaque);
+
+        save_section_footer(f, se);
+
+        if (ret < 0) {
+            qemu_file_set_error(f, ret);
+            return -1;
+        }
+    }
+
     QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
         if (!se->ops ||
             (in_postcopy && se->ops->has_postcopy &&
@@ -1528,6 +1549,20 @@ int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
                                     end_ts_each - start_ts_each);
     }
 
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (!se->ops || (in_postcopy && se->ops->has_postcopy &&
+             se->ops->has_postcopy(se->opaque)) ||
+            !se->ops->save_live_complete_precopy_async_wait) {
+            continue;
+        }
+
+        ret = se->ops->save_live_complete_precopy_async_wait(f, se->opaque);
+        if (ret < 0) {
+            qemu_file_set_error(f, ret);
+            return -1;
+        }
+    }
+
     trace_vmstate_downtime_checkpoint("src-iterable-saved");
 
     return 0;