[virtio-comment] Re: [PATCH v2 7/7] virtio-net: Describe RSS using receive queue handle

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

On Tue, Mar 21, 2023 at 06:10:36AM +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>
> 
> ---
> changelog:
> v0->v1:
> - new patch suggested by Michael Tsirkin
> ---
>  device-types/net/description.tex | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index 435c1fc..fa9b4c7 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -1442,20 +1442,32 @@ \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 \field{unclassified_queue} contains the receive queue
> +handle \field{rq_handle} described below.
>  
> -Field \field{indirection_table} contains an array of 0-based indices of receive virtqueues. Index 0 corresponds to receiveq1.
> +Field \field{indirection_table} contains an array of receive
> +queue handles described below in \field{rq_handle}.


This is only used once, here.  I understand that we need a name
for this but it need not be so generic, I feel it's just confusing.
I propose "indirection table entry".

Thus:
	Field \field{indirection_table} contains an array of
	indirection table entries. Each indirection table entry consists of a
	vq number value divided by 2. Thus, an entry value of 3 maps to virtqueue number 6
	which corresponds to receiveq4



> For example at
> +array index 0,

why mention array index here?

> a value of 3 maps to virtqueue number 6
> +which corresponds to receiveq4.

It's a good idea to first describe a concept then provide
an example.


>  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}).
>  
>  Fields \field{hash_key_length} and \field{hash_key_data} define the key to be used in hash calculation.
>  
> +\begin{lstlisting}
> +le16 rq_handle;
> +\end{lstlisting}
> +
> +\field{rq_handle} is a receive virtqueue handle. It is calculated as
> +virtqueue number divided by 2. For example a receive virtqueue handle
> +value of 3 corresponds to virtqueue number 6 maps to receiveq4.
> +

it's a bit disconnected from the rest of the description. why not
adjacent to it?

>  \drivernormative{\subparagraph}{Setting RSS parameters}{Device Types / Network Device / Device Operation / Control Virtqueue / Receive-side scaling (RSS) }
>  
>  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{rq_handle} corresponding to the enabled receive virtqueues.
>  
>  The number of entries in \field{indirection_table} (\field{indirection_table_mask} + 1) MUST be a power of two.
>  
> @@ -1468,7 +1480,9 @@ \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{rq_handle}.
>  \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


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