virtio-msg: introduce a message correlation protocol

Add a sub-section to do a first explanation draft of how the message
correlation is to be done using a combination of a message device number
and the message unique identifier value.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>

Author: bertrand-marquis

transport-msg.tex

@@ -332,6 +332,29 @@ \subsubsection{Common Message Format}
 sent as zero and \textbf{MUST} be ignored on receive to preserve forward
 compatibility.
 
+\subsubsection{Message Correlation}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Correlation}
+
+Correlation associates a response with its request:
+\begin{itemize}
+  \item Bus Message key: \field{msg_uid}.
+  \item Transport Message key: (\field{dev_num}, \field{msg_uid}).
+\end{itemize}
+
+Rules:
+\begin{itemize}
+  \item The driver allocates a non-zero \field{msg_uid} for every request that
+    expects a response and \textbf{MUST NOT} reuse it while in flight.
+  \item Event (one-way) messages \textbf{MUST} set \field{msg_uid}=0 and MUST
+    NOT yield a response.
+  \item A response \textbf{MUST} echo \field{msg_uid}, \field{dev_num} (for
+    transport), and \field{msg_op}.
+  \item Unknown or already completed correlation tuples \textbf{SHOULD} result
+    in discarding the response without further protocol action.
+  \item Implementations \textbf{MAY} deliver responses out of order unless a
+    specific message definition mandates ordering.
+\end{itemize}
+
 \subsection{Bus Operation}
 \label{sec:Virtio Transport Options / Virtio Over Messages / Bus Operation}