ntb_msg_read
val = ntb_msg_read(perf->ntb, pidx, PERF_MSG_CMD);
val = ntb_msg_read(perf->ntb, pidx, PERF_MSG_LDATA);
val = ntb_msg_read(perf->ntb, pidx, PERF_MSG_HDATA);
msg_data = ntb_msg_read(pp->ntb, &pidx, 0);
data = ntb_msg_read(msg->tc->ntb, &pidx, msg->midx);