RE: [virtio-dev] [PATCH] virtio-net: Fix and update VIRTIO_NET_F_NOTF_COAL feature

[Parav Pandit <parav@nvidia.com>]

> From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> Behalf Of Alvaro Karsz
> 
> This patch makes several improvements to the notification coalescing feature,
> including:
> 
> - Consolidating virtio_net_ctrl_coal_tx and virtio_net_ctrl_coal_rx
>   into a single struct, virtio_net_ctrl_coal, as they are identical.
> - Emphasizing that the coalescing commands are best-effort.
> - Defining the behavior of coalescing with regards to delivering
>   notifications when a change occur.
>
Patch needs to do one thing at a time.
Please split above into three patches.
 
> Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
> ---
>  device-types/net/description.tex | 40 ++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 18 deletions(-)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index 1741c79..2a98411 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -1514,15 +1514,12 @@ \subsubsection{Control
> Virtqueue}\label{sec:Device Types / Network Device / Devi  If the
> VIRTIO_NET_F_NOTF_COAL feature is negotiated, the driver can  send control
> commands for dynamically changing the coalescing parameters.
> 
> -\begin{lstlisting}
> -struct virtio_net_ctrl_coal_rx {
> -    le32 rx_max_packets;
> -    le32 rx_usecs;
> -};
> +Note: In general, these commands are best-effort: A device could send a
> notification even if it is not supposed to.
>
Please remove this note from this patch.
Instead of Note, we need to describe this device requirements description.
We better need to describe the motivation for it.
We may want to say there may be jitter in notification, but device should not be sending when it is not supposed to.

I also have more description to add in this area with regards to GSO and LRO.
I make humble suggestion that we draft is jointly in separate patch combining these clarifications.
 
> -struct virtio_net_ctrl_coal_tx {
> -    le32 tx_max_packets;
> -    le32 tx_usecs;
> +\begin{lstlisting}
> +struct virtio_net_ctrl_coal {
> +    le32 max_packets;
> +    le32 usecs;
>  };
> 
This is one good change to go as separate patch.

>  #define VIRTIO_NET_CTRL_NOTF_COAL 6
> @@ -1532,25 +1529,25 @@ \subsubsection{Control
> Virtqueue}\label{sec:Device Types / Network Device / Devi
> 
>  Coalescing parameters:
>  \begin{itemize}
> -\item \field{rx_usecs}: Maximum number of usecs to delay a RX notification.
> -\item \field{tx_usecs}: Maximum number of usecs to delay a TX notification.
> -\item \field{rx_max_packets}: Maximum number of packets to receive before
> a RX notification.
> -\item \field{tx_max_packets}: Maximum number of packets to send before a
> TX notification.
> +\item \field{usecs} for RX: Maximum number of usecs to delay a RX
> notification.
> +\item \field{usecs} for TX: Maximum number of usecs to delay a TX
> notification.
> +\item \field{max_packets} for RX: Maximum number of packets to receive
> before a RX notification.
> +\item \field{max_packets} for TX: Maximum number of packets to send before
> a TX notification.
>  \end{itemize}
> 
s/for Rx/For receive virtqueue
s/for Tx/For transmit virtqueue
> 
>  The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
>  \begin{enumerate}
> -\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{tx_usecs} and
> \field{tx_max_packets} parameters.
> -\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{rx_usecs} and
> \field{rx_max_packets} parameters.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{usecs} and
> \field{max_packets} parameters for TX.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{usecs} and
> \field{max_packets} parameters for RX.
>  \end{enumerate}
> 
>  \subparagraph{RX Notifications}\label{sec:Device Types / Network Device /
> Device Operation / Control Virtqueue / Notifications Coalescing / RX
> Notifications}
> 
>  If, for example:
>  \begin{itemize}
> -\item \field{rx_usecs} = 10.
> -\item \field{rx_max_packets} = 15.
> +\item \field{usecs} = 10.
> +\item \field{max_packets} = 15.
>  \end{itemize}
> 
>  The device will operate as follows:
> @@ -1564,8 +1561,8 @@ \subsubsection{Control Virtqueue}\label{sec:Device
> Types / Network Device / Devi
> 
>  If, for example:
>  \begin{itemize}
> -\item \field{tx_usecs} = 10.
> -\item \field{tx_max_packets} = 15.
> +\item \field{usecs} = 10.
> +\item \field{max_packets} = 15.
>  \end{itemize}
> 
>  The device will operate as follows:
> @@ -1575,6 +1572,13 @@ \subsubsection{Control
> Virtqueue}\label{sec:Device Types / Network Device / Devi  \item If the
> notifications are not suppressed by the driver, the device will send an used
> buffer notification, otherwise, the device will not send an used buffer
> notification as long as the notifications are suppressed.
>  \end{itemize}
> 
> +\subparagraph{Notifications When Coalescing Parameters
> +Change}\label{sec:Device Types / Network Device / Device Operation /
> +Control Virtqueue / Notifications Coalescing / Notifications When
> +Coalescing Parameters Change}
> +
> +When a device changes the coalescing parameters, the device needs to check
> if the new parameters are met and issue a notification if so.
> +
> +For example, \field{max_packets} = 15 for TX.
> +If the device sends 10 packets, then it receives a
> VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command with \field{max_packets} =
> 8, the device needs to immediately send a TX notification, if the notifications
> are not suppressed by the driver.
> +
Its better written as,
When device applies new coalescing parameters, if the new parameters already meet the current packet counters, device SHOULD generate immediate notification.

For example, current \field{max_packets} is 15 for transmit virtqueue, and device already sent 10 packets.
When device receives a VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, the device SHOULD immediately send a TX notification.

Above clarification also should be in same patch series as second patch.

>  \drivernormative{\subparagraph}{Notifications Coalescing}{Device Types /
> Network Device / Device Operation / Control Virtqueue / Notifications
> Coalescing}
> 
>  If the VIRTIO_NET_F_NOTF_COAL feature has not been negotiated, the driver
> MUST NOT issue VIRTIO_NET_CTRL_NOTF_COAL commands.
> --
> 2.34.1
> 
> 
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