migration: Add save_live_complete_precopy_thread handler

This SaveVMHandler helps device provide its own asynchronous transmission
of the remaining data at the end of a precopy phase via multifd channels,
in parallel with the transfer done by save_live_complete_precopy handlers.

These threads are launched only when multifd device state transfer is
supported.

Management of these threads in done in the multifd migration code,
wrapping them in the generic thread pool.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/qemu-devel/eac74a4ca7edd8968bbf72aa07b9041c76364a16.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

3 files changed, 39 insertions, 0 deletions

diff --git a/include/migration/misc.h b/include/migration/misc.h
index 273ebfc..8fd36eb 100644
--- a/include/migration/misc.h
+++ b/include/migration/misc.h
@@ -119,8 +119,25 @@ bool migrate_uri_parse(const char *uri, MigrationChannel **channel,
                        Error **errp);
 
 /* migration/multifd-device-state.c */
+typedef struct SaveLiveCompletePrecopyThreadData {
+    SaveLiveCompletePrecopyThreadHandler hdlr;
+    char *idstr;
+    uint32_t instance_id;
+    void *handler_opaque;
+} SaveLiveCompletePrecopyThreadData;
+
 bool multifd_queue_device_state(char *idstr, uint32_t instance_id,
                                 char *data, size_t len);
 bool multifd_device_state_supported(void);
 
+void
+multifd_spawn_device_state_save_thread(SaveLiveCompletePrecopyThreadHandler hdlr,
+                                       char *idstr, uint32_t instance_id,
+                                       void *opaque);
+
+bool multifd_device_state_save_thread_should_exit(void);
+
+void multifd_abort_device_state_save_threads(void);
+bool multifd_join_device_state_save_threads(void);
+
 #endif
diff --git a/include/migration/register.h b/include/migration/register.h
index 58891aa..c041ce3 100644
--- a/include/migration/register.h
+++ b/include/migration/register.h
@@ -105,6 +105,25 @@ typedef struct SaveVMHandlers {
      */
     int (*save_live_complete_precopy)(QEMUFile *f, void *opaque);
 
+    /**
+     * @save_live_complete_precopy_thread (invoked in a separate thread)
+     *
+     * Called at the end of a precopy phase from a separate worker thread
+     * in configurations where multifd device state transfer is supported
+     * in order to perform asynchronous transmission of the remaining data in
+     * parallel with @save_live_complete_precopy handlers.
+     * When postcopy is enabled, devices that support postcopy will skip this
+     * step.
+     *
+     * @d: a #SaveLiveCompletePrecopyThreadData containing parameters that the
+     * handler may need, including this device section idstr and instance_id,
+     * and opaque data pointer passed to register_savevm_live().
+     * @errp: pointer to Error*, to store an error if it happens.
+     *
+     * Returns true to indicate success and false for errors.
+     */
+    SaveLiveCompletePrecopyThreadHandler save_live_complete_precopy_thread;
+
     /* This runs both outside and inside the BQL.  */
 
     /**
diff --git a/include/qemu/typedefs.h b/include/qemu/typedefs.h
index fd23ff7..42ed4e6 100644
--- a/include/qemu/typedefs.h
+++ b/include/qemu/typedefs.h
@@ -108,6 +108,7 @@ typedef struct QString QString;
 typedef struct RAMBlock RAMBlock;
 typedef struct Range Range;
 typedef struct ReservedRegion ReservedRegion;
+typedef struct SaveLiveCompletePrecopyThreadData SaveLiveCompletePrecopyThreadData;
 typedef struct SHPCDevice SHPCDevice;
 typedef struct SSIBus SSIBus;
 typedef struct TCGCPUOps TCGCPUOps;
@@ -133,5 +134,7 @@ typedef struct IRQState *qemu_irq;
 typedef void (*qemu_irq_handler)(void *opaque, int n, int level);
 typedef bool (*MigrationLoadThread)(void *opaque, bool *should_quit,
                                     Error **errp);
+typedef bool (*SaveLiveCompletePrecopyThreadHandler)(SaveLiveCompletePrecopyThreadData *d,
+                                                     Error **errp);
 
 #endif /* QEMU_TYPEDEFS_H */