vfio/migration: Add vfio_save_block_precopy_empty_hit trace event

This way it is clearly known when there's no more data to send for that
device.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

hw/vfio/migration.c

@@ -370,6 +370,10 @@ static ssize_t vfio_save_block(QEMUFile *f, VFIOMigration *migration)
          * please refer to the Linux kernel VFIO uAPI.
          */
         if (errno == ENOMSG) {
+            if (!migration->event_precopy_empty_hit) {
+                trace_vfio_save_block_precopy_empty_hit(migration->vbasedev->name);
+                migration->event_precopy_empty_hit = true;
+            }
             return 0;
         }
 
@@ -379,6 +383,9 @@ static ssize_t vfio_save_block(QEMUFile *f, VFIOMigration *migration)
         return 0;
     }
 
+    /* Non-empty read: re-arm the trace event */
+    migration->event_precopy_empty_hit = false;
+
     qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
     qemu_put_be64(f, data_size);
     qemu_put_buffer(f, migration->data_buffer, data_size);
@@ -473,6 +480,7 @@ static int vfio_save_setup(QEMUFile *f, void *opaque, Error **errp)
     }
 
     migration->event_save_iterate_started = false;
+    migration->event_precopy_empty_hit = false;
 
     if (vfio_precopy_supported(vbasedev)) {
         switch (migration->device_state) {

hw/vfio/trace-events

@@ -157,6 +157,7 @@ vfio_migration_set_device_state(const char *name, const char *state) " (%s) stat
 vfio_migration_set_state(const char *name, const char *new_state, const char *recover_state) " (%s) new state %s, recover state %s"
 vfio_migration_state_notifier(const char *name, int state) " (%s) state %d"
 vfio_save_block(const char *name, int data_size) " (%s) data_size %d"
+vfio_save_block_precopy_empty_hit(const char *name) " (%s)"
 vfio_save_cleanup(const char *name) " (%s)"
 vfio_save_complete_precopy(const char *name, int ret) " (%s) ret %d"
 vfio_save_complete_precopy_start(const char *name) " (%s)"

include/hw/vfio/vfio-common.h

@@ -75,6 +75,7 @@ typedef struct VFIOMigration {
     bool initial_data_sent;
 
     bool event_save_iterate_started;
+    bool event_precopy_empty_hit;
 } VFIOMigration;
 
 struct VFIOGroup;