The transfer of an I/O read request on the completer request interface is similar to that
of a memory read request, and involves only the descriptor. The length of the requested
data is always one Dword, and the First Byte Enable bits in
m_axis_cq_tuser indicate the valid bytes to be read.
The user logic must respond to an I/O read request with a one-Dword Completion (or a Completion with no data in the case of an error). The Completions for two distinct I/O read requests are be sent in any order. Errors associated with an I/O read transaction can be signaled to the requester by setting the Completion Status field in the completion descriptor to CA (Completer Abort) or UR (Unsupported Request), as is appropriate. The operation of the completer completion interface is described in 64/128/256-bit Completer Interface and 512-bit Completer Interface.