Re: [PATCH v4 8/8] virtio-net: Describe RSS using receive queue handle

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 22, 2023 at 11:52:43PM +0200, Parav Pandit wrote:
> The content of indirection table and unclassified_queue which are
> based on math calculation historically. To better describe this, to
> avoid intermixing array index with virtqueue index and to use virtqueue
> number
> 
> introduce a field rq_handle (receive queue handle) and refer them
> to describe unclassified_queue and indirection_table fields.
> 
> As part of it, have the example that uses non zero virtqueue
> number which helps to have better mapping between receiveX
> object with virtqueue number 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>

getting there, I feel however there's still too much focus on
rq_id. It's just a struct we use. What matters is the vqn.
More comments below.

> ---
> changelog:
> 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 435c1fc..8b560f3 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -1423,11 +1423,16 @@ \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 vqn_1_16: 15; /* Bits 1 to 16 of the virtqueue number */
> +   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];
> @@ -1442,10 +1447,15 @@ \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{vqn_1_16}

Looks like you keep removing the main thing which is *what is this field
actually for*.

More comments:

id is actually abbreviation. Field is actually \field{unclassified_queue} -
rss_rq_id is just a name of struct has no meaning by itself.
so either \field{struct rss_rq_id} or just don't mention this at all.
Below example on how not to mention at all.

Here's one way to put it:

\field{unclassified_queue} identifies a receive virtqueue
to place unclassified packets in. Within \field{unclassified_queue}, \field{vqn_1_16},
consists of bits 1 to 16 of the receive virtqueue number
For example,
\field{vqn_1_16} value of 3 corresponds to virtqueue number 6 maps to receiveq4.

> +consists of bits 1 to 16 of virtqueue number. For example,
> +\field{vqn_1_16} value of 3 corresponds to virtqueue number 6 maps to receiveq4.
> +
> +Field \field{unclassified_queue} contains the receive virtqueue
> +id \field{rss_rq_id} described above.
>  
> -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
> +queue ids described above in \field{rss_rq_id}.

Field \field{indirection_table} is an array of receive queue
identifiers, using the same format as \field{unclassified_queue}.


>  
>  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}).
>  
> @@ -1455,7 +1465,8 @@ \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
> +the \field{rss_rq_id} corresponding to the enabled receive virtqueues.

There's no field like this. Also not "the" virtqueues - we didn't
discuss them previously. Generally if you want to tweak grammar it's
best to do it separately...

example way to put it better:

\field{vqn_1_16} in each entry corresponding to enabled receive virtqueues.





>  
>  The number of entries in \field{indirection_table} (\field{indirection_table_mask} + 1) MUST be a power of two.
>  
> @@ -1468,7 +1479,9 @@ \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
> +\field{rss_rq_id}.

to get \field{vqn_1_16}

>  \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