migration: Add documentation for SaveVMHandlers

The SaveVMHandlers structure is still in use for complex subsystems
and devices. Document the handlers since we are going to modify a few
later.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240304122844.1888308-9-clg@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

include/migration/register.h

@@ -16,30 +16,130 @@
 
 #include "hw/vmstate-if.h"
 
+/**
+ * struct SaveVMHandlers: handler structure to finely control
+ * migration of complex subsystems and devices, such as RAM, block and
+ * VFIO.
+ */
 typedef struct SaveVMHandlers {
-    /* This runs inside the BQL.  */
+
+    /* The following handlers run inside the BQL. */
+
+    /**
+     * @save_state
+     *
+     * Saves state section on the source using the latest state format
+     * version.
+     *
+     * Legacy method. Should be deprecated when all users are ported
+     * to VMStateDescription.
+     *
+     * @f: QEMUFile where to send the data
+     * @opaque: data pointer passed to register_savevm_live()
+     */
     void (*save_state)(QEMUFile *f, void *opaque);
 
-    /*
+    /**
-     * save_prepare is called early, even before migration starts, and can be
+     * @save_prepare
-     * used to perform early checks.
+     *
+     * Called early, even before migration starts, and can be used to
+     * perform early checks.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     * @errp: pointer to Error*, to store an error if it happens.
+     *
+     * Returns zero to indicate success and negative for error
      */
     int (*save_prepare)(void *opaque, Error **errp);
+
+    /**
+     * @save_setup
+     *
+     * Initializes the data structures on the source and transmits
+     * first section containing information on the device
+     *
+     * @f: QEMUFile where to send the data
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*save_setup)(QEMUFile *f, void *opaque);
+
+    /**
+     * @save_cleanup
+     *
+     * Uninitializes the data structures on the source
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     */
     void (*save_cleanup)(void *opaque);
+
+    /**
+     * @save_live_complete_postcopy
+     *
+     * Called at the end of postcopy for all postcopyable devices.
+     *
+     * @f: QEMUFile where to send the data
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*save_live_complete_postcopy)(QEMUFile *f, void *opaque);
+
+    /**
+     * @save_live_complete_precopy
+     *
+     * Transmits the last section for the device containing any
+     * remaining data at the end of a precopy phase. When postcopy is
+     * enabled, devices that support postcopy will skip this step,
+     * where the final data will be flushed at the end of postcopy via
+     * @save_live_complete_postcopy instead.
+     *
+     * @f: QEMUFile where to send the data
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*save_live_complete_precopy)(QEMUFile *f, void *opaque);
 
     /* This runs both outside and inside the BQL.  */
+
+    /**
+     * @is_active
+     *
+     * Will skip a state section if not active
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns true if state section is active else false
+     */
     bool (*is_active)(void *opaque);
+
+    /**
+     * @has_postcopy
+     *
+     * Checks if a device supports postcopy
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns true for postcopy support else false
+     */
     bool (*has_postcopy)(void *opaque);
 
-    /* is_active_iterate
+    /**
-     * If it is not NULL then qemu_savevm_state_iterate will skip iteration if
+     * @is_active_iterate
-     * it returns false. For example, it is needed for only-postcopy-states,
+     *
-     * which needs to be handled by qemu_savevm_state_setup and
+     * As #SaveVMHandlers.is_active(), will skip an inactive state
-     * qemu_savevm_state_pending, but do not need iterations until not in
+     * section in qemu_savevm_state_iterate.
-     * postcopy stage.
+     *
+     * For example, it is needed for only-postcopy-states, which needs
+     * to be handled by qemu_savevm_state_setup() and
+     * qemu_savevm_state_pending(), but do not need iterations until
+     * not in postcopy stage.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns true if state section is active else false
      */
     bool (*is_active_iterate)(void *opaque);
 
@@ -48,44 +148,155 @@ typedef struct SaveVMHandlers {
      * use data that is local to the migration thread or protected
      * by other locks.
      */
+
+    /**
+     * @save_live_iterate
+     *
+     * Should send a chunk of data until the point that stream
+     * bandwidth limits tell it to stop. Each call generates one
+     * section.
+     *
+     * @f: QEMUFile where to send the data
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns 0 to indicate that there is still more data to send,
+     *         1 that there is no more data to send and
+     *         negative to indicate an error.
+     */
     int (*save_live_iterate)(QEMUFile *f, void *opaque);
 
     /* This runs outside the BQL!  */
-    /* Note for save_live_pending:
+
-     * must_precopy:
+    /**
-     * - must be migrated in precopy or in stopped state
+     * @state_pending_estimate
-     * - i.e. must be migrated before target start
      *
-     * can_postcopy:
+     * This estimates the remaining data to transfer
-     * - can migrate in postcopy or in stopped state
-     * - i.e. can migrate after target start
-     * - some can also be migrated during precopy (RAM)
-     * - some must be migrated after source stops (block-dirty-bitmap)
      *
-     * Sum of can_postcopy and must_postcopy is the whole amount of
+     * Sum of @can_postcopy and @must_postcopy is the whole amount of
      * pending data.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     * @must_precopy: amount of data that must be migrated in precopy
+     *                or in stopped state, i.e. that must be migrated
+     *                before target start.
+     * @can_postcopy: amount of data that can be migrated in postcopy
+     *                or in stopped state, i.e. after target start.
+     *                Some can also be migrated during precopy (RAM).
+     *                Some must be migrated after source stops
+     *                (block-dirty-bitmap)
      */
-    /* This estimates the remaining data to transfer */
     void (*state_pending_estimate)(void *opaque, uint64_t *must_precopy,
                                    uint64_t *can_postcopy);
-    /* This calculate the exact remaining data to transfer */
+
+    /**
+     * @state_pending_exact
+     *
+     * This calculates the exact remaining data to transfer
+     *
+     * Sum of @can_postcopy and @must_postcopy is the whole amount of
+     * pending data.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     * @must_precopy: amount of data that must be migrated in precopy
+     *                or in stopped state, i.e. that must be migrated
+     *                before target start.
+     * @can_postcopy: amount of data that can be migrated in postcopy
+     *                or in stopped state, i.e. after target start.
+     *                Some can also be migrated during precopy (RAM).
+     *                Some must be migrated after source stops
+     *                (block-dirty-bitmap)
+     */
     void (*state_pending_exact)(void *opaque, uint64_t *must_precopy,
                                 uint64_t *can_postcopy);
+
+    /**
+     * @load_state
+     *
+     * Load sections generated by any of the save functions that
+     * generate sections.
+     *
+     * Legacy method. Should be deprecated when all users are ported
+     * to VMStateDescription.
+     *
+     * @f: QEMUFile where to receive the data
+     * @opaque: data pointer passed to register_savevm_live()
+     * @version_id: the maximum version_id supported
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*load_state)(QEMUFile *f, void *opaque, int version_id);
+
+    /**
+     * @load_setup
+     *
+     * Initializes the data structures on the destination.
+     *
+     * @f: QEMUFile where to receive the data
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*load_setup)(QEMUFile *f, void *opaque);
+
+    /**
+     * @load_cleanup
+     *
+     * Uninitializes the data structures on the destination.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*load_cleanup)(void *opaque);
-    /* Called when postcopy migration wants to resume from failure */
+
+    /**
+     * @resume_prepare
+     *
+     * Called when postcopy migration wants to resume from failure
+     *
+     * @s: Current migration state
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
     int (*resume_prepare)(MigrationState *s, void *opaque);
-    /* Checks if switchover ack should be used. Called only in dest */
+
+    /**
+     * @switchover_ack_needed
+     *
+     * Checks if switchover ack should be used. Called only on
+     * destination.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns true if switchover ack should be used and false
+     * otherwise
+     */
     bool (*switchover_ack_needed)(void *opaque);
 } SaveVMHandlers;
 
+/**
+ * register_savevm_live: Register a set of custom migration handlers
+ *
+ * @idstr: state section identifier
+ * @instance_id: instance id
+ * @version_id: version id supported
+ * @ops: SaveVMHandlers structure
+ * @opaque: data pointer passed to SaveVMHandlers handlers
+ */
 int register_savevm_live(const char *idstr,
                          uint32_t instance_id,
                          int version_id,
                          const SaveVMHandlers *ops,
                          void *opaque);
 
+/**
+ * unregister_savevm: Unregister custom migration handlers
+ *
+ * @obj: object associated with state section
+ * @idstr:  state section identifier
+ * @opaque: data pointer passed to register_savevm_live()
+ */
 void unregister_savevm(VMStateIf *obj, const char *idstr, void *opaque);
 
 #endif