[PATCH v4 2/8] admin: Redefine reserved2 as command specific output

[Parav Pandit <parav@nvidia.com>]

Currently when a command wants to get two distinct types of data in
the result, such as one consumed by the driver, other to be zero
copied to some user buffers, the driver needs to prepare an
extra descriptor for driver consumed field. When such a field is
<= 4 bytes, extra descriptor is an overhead.

virtio_admin_command already has 4B of reserved for the device
writable area. Utilize it to define as device writable output.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/176
Signed-off-by: Parav Pandit <parav@nvidia.com>
---
 admin.tex | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/admin.tex b/admin.tex
index 6eeef58..c86813d 100644
--- a/admin.tex
+++ b/admin.tex
@@ -90,8 +90,7 @@ \subsection{Group administration commands}\label{sec:Basic Facilities of a Virti
         /* Device-writable part */
         le16 status;
         le16 status_qualifier;
-        /* unused, reserved for future extensions */
-        u8 reserved2[4];
+        u8 command_specific_output[4];
         u8 command_specific_result[];
 };
 \end{lstlisting}
@@ -192,11 +191,15 @@ \subsection{Group administration commands}\label{sec:Basic Facilities of a Virti
 \hline
 \end{tabularx}
 
-Each command uses a different \field{command_specific_data} and
-\field{command_specific_result} structures and the length of
+Each command uses a different \field{command_specific_data},
+\field{command_specific_output} and
+\field{command_specific_result} fields. The length of
 \field{command_specific_data} and \field{command_specific_result}
-depends on these structures and is described separately or is
-implicit in the structure description.
+depends on the command and is described separately or is
+implicit in the structure description. The \field{command_specific_output}
+describes any command specific output which is up to 4 bytes size. The
+\field{command_specific_output} contain one or more command specific
+fields.
 
 Before sending any group administration commands to the device, the driver
 needs to communicate to the device which commands it is going to
-- 
2.34.1