MAX_DDP_BUFFER_SIZE
if (end - start > MAX_DDP_BUFFER_SIZE) {
end = rounddown2(start + MAX_DDP_BUFFER_SIZE, CACHE_LINE_SIZE);
if (t4_ddp_rcvbuf_len > MAX_DDP_BUFFER_SIZE)
t4_ddp_rcvbuf_len = MAX_DDP_BUFFER_SIZE;