tracing_map_read_var
var_val = tracing_map_read_var(var_elt, var_idx);
track_val = tracing_map_read_var(elt, track_var_idx);
val = tracing_map_read_var(elt, save_var_idx);
extern u64 tracing_map_read_var(struct tracing_map_elt *elt, unsigned int i);