[PATCH v15 10/11] virtio-net: Describe RSS using rss rq id

[Parav Pandit <parav@nvidia.com>]

The content of the indirection table and unclassified_queue were
originally described based on mathematical operations. In order to
make it easier to understand and to avoid intermixing the array
index with the vq index, introduce a structure
rss_rq_id (RSS receive queue
ID) and use it to describe the unclassified_queue and
indirection_table fields.

As part of it, have the example that uses non-zero virtqueue
index which helps to have better mapping between receiveX
object with virtqueue index and the actual value in the
indirection table.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/163
Signed-off-by: Parav Pandit <parav@nvidia.com>

---
changelog:
v14->v15:
- address next comment from Michael and Halil
- changed description of unclassified_queue to use 'specifies'
v12->v13:
- rename vqn to vq_index
- renamed vq index to virtqueue index
v11->v12:
- use 'virtqueue index' instead of 'virtqueue number'
v10->v11:
- commit log updated to drop old reference to rq_handle, replaced with
  rss_rq_id detail
v8->v9:
- reword rss_rq_id and unclassified_packets description
- use article
- use 'vq number' instead of 'virtqueue number'
v4->v5:
- change subject to reflect rss_rq_id
- fixed accidental removal of "unclassifed packets".
- simplfied text around indirection_table mask to use the term
  destination receive virtqueue. This aligns with next line about queue
  reset.
- removed rss_rq_id references as indirection table and
  unclassified_queue data type is self explanatory
v3->v4:
- renamed rq_handle to rss_rq_id
- moved rss_rq_id definition close to its usage in rss_config struct
v2->v3:
- moved rq_handle definition before using it
- removed duplicate example as rq_handle is now described first
v0->v1:
- new patch suggested by Michael Tsirkin
---
 device-types/net/description.tex | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index f3f9f1d..e25a9d0 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -1434,11 +1434,16 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 
 Driver sends a VIRTIO_NET_CTRL_MQ_RSS_CONFIG command using the following format for \field{command-specific-data}:
 \begin{lstlisting}
+struct rss_rq_id {
+   le16 vq_index_1_16: 15; /* Bits 1 to 16 of the virtqueue index */
+   le16 reserved: 1; /* Set to zero */
+};
+
 struct virtio_net_rss_config {
     le32 hash_types;
     le16 indirection_table_mask;
-    le16 unclassified_queue;
-    le16 indirection_table[indirection_table_length];
+    struct rss_rq_id unclassified_queue;
+    struct rss_rq_id indirection_table[indirection_table_length];
     le16 max_tx_vq;
     u8 hash_key_length;
     u8 hash_key_data[hash_key_length];
@@ -1453,10 +1458,15 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 \field{indirection_table} array.
 Number of entries in \field{indirection_table} is (\field{indirection_table_mask} + 1).
 
-Field \field{unclassified_queue} contains the 0-based index of
-the receive virtqueue to place unclassified packets in. Index 0 corresponds to receiveq1.
+\field{rss_rq_id} is a receive virtqueue id. \field{vq_index_1_16}
+consists of bits 1 to 16 of a virtqueue index. For example, a
+\field{vq_index_1_16} value of 3 corresponds to virtqueue index 6,
+which maps to receiveq4.
+
+Field \field{unclassified_queue} specifies the receive virtqueue in which to
+place unclassified packets.
 
-Field \field{indirection_table} contains an array of 0-based indices of receive virtqueues. Index 0 corresponds to receiveq1.
+Field \field{indirection_table} is an array of receive virtqueues.
 
 A driver sets \field{max_tx_vq} to inform a device how many transmit virtqueues it may use (transmitq1\ldots transmitq \field{max_tx_vq}).
 
@@ -1466,7 +1476,8 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 
 A driver MUST NOT send the VIRTIO_NET_CTRL_MQ_RSS_CONFIG command if the feature VIRTIO_NET_F_RSS has not been negotiated.
 
-A driver MUST fill the \field{indirection_table} array only with indices of enabled queues. Index 0 corresponds to receiveq1.
+A driver MUST fill the \field{indirection_table} array only with
+enabled receive virtqueues.
 
 The number of entries in \field{indirection_table} (\field{indirection_table_mask} + 1) MUST be a power of two.
 
@@ -1479,7 +1490,9 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 \begin{itemize}
 \item Calculate the hash of the packet as defined in \ref{sec:Device Types / Network Device / Device Operation / Processing of Incoming Packets / Hash calculation for incoming packets}.
 \item If the device did not calculate the hash for the specific packet, the device directs the packet to the receiveq specified by \field{unclassified_queue} of virtio_net_rss_config structure.
-\item Apply \field{indirection_table_mask} to the calculated hash and use the result as the index in the indirection table to get 0-based number of destination receiveq.
+\item Apply \field{indirection_table_mask} to the calculated hash
+and use the result as the index in the indirection table to get
+the destination receive virtqueue.
 \item If the destination receive queue is being reset (See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}), the device MUST drop the packet.
 \end{itemize}
 
-- 
2.26.2