vfio/migration: Refactor vfio_save_block() to return saved data size

Refactor vfio_save_block() to return the size of saved data on success
and -errno on error.

This will be used in next patch to implement VFIO migration pre-copy
support.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/migration.c

@@ -241,8 +241,8 @@ static int vfio_query_stop_copy_size(VFIODevice *vbasedev,
     return 0;
 }
 
-/* Returns 1 if end-of-stream is reached, 0 if more data and -errno if error */
-static int vfio_save_block(QEMUFile *f, VFIOMigration *migration)
+/* Returns the size of saved data on success and -errno on error */
+static ssize_t vfio_save_block(QEMUFile *f, VFIOMigration *migration)
 {
     ssize_t data_size;
 
@@ -252,7 +252,7 @@ static int vfio_save_block(QEMUFile *f, VFIOMigration *migration)
         return -errno;
     }
     if (data_size == 0) {
-        return 1;
+        return 0;
     }
 
     qemu_put_be64(f, VFIO_MIG_FLAG_DEV_DATA_STATE);
@@ -262,7 +262,7 @@ static int vfio_save_block(QEMUFile *f, VFIOMigration *migration)
 
     trace_vfio_save_block(migration->vbasedev->name, data_size);
 
-    return qemu_file_get_error(f);
+    return qemu_file_get_error(f) ?: data_size;
 }
 
 /* ---------------------------------------------------------------------- */
@@ -335,6 +335,7 @@ static void vfio_state_pending_exact(void *opaque, uint64_t *must_precopy,
 static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
 {
     VFIODevice *vbasedev = opaque;
+    ssize_t data_size;
     int ret;
 
     /* We reach here with device state STOP only */
@@ -345,11 +346,11 @@ static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)
     }
 
     do {
-        ret = vfio_save_block(f, vbasedev->migration);
-        if (ret < 0) {
-            return ret;
+        data_size = vfio_save_block(f, vbasedev->migration);
+        if (data_size < 0) {
+            return data_size;
         }
-    } while (!ret);
+    } while (data_size);
 
     qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
     ret = qemu_file_get_error(f);