The transfer of a message on the requester request interface is similar to that of a
memory write request, except that a payload are not always be present. The transfer
starts with the 128-bit descriptor, followed by the payload, if present. The first Dword
of the payload must immediately follow the descriptor, regardless of the address
alignment mode in use. The addr_offset[3:0] field in the
s_axis_rq_tuser bus must be set to 0 for messages when the
address-aligned mode is in use. The core determines the end of the payload from
s_axis_rq_tlast and s_axis_rq_tkeep signals. The
First Byte Enable and Last Byte Enable bits (first_be[7:0] and
last_be[7:0]) are not used for message requests.