[virtio-dev] Re: [virtio-comment] [PATCH v10] virtio-net: support the virtqueue coalescing moderation

[David Edmondson <david.edmondson@oracle.com>]

On Wednesday, 2023-03-01 at 22:10:27 +08, Heng Qi wrote:
> Currently, coalescing parameters are grouped for all transmit and receive
> virtqueues. This patch supports setting or getting the parameters for a
> specified virtqueue, and a typical application of this function is netdim[1].
>
> When the traffic between virtqueues is unbalanced, for example, one virtqueue
> is busy and another virtqueue is idle, then it will be very useful to
> control coalescing parameters at the virtqueue granularity.
>
> [1] https://docs.kernel.org/networking/net_dim.html
>
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> This patch is on top of Alvaro's latest v7 patch: https://lists.oasis-open.org/archives/virtio-dev/202302/msg00431.html .
>
> v9->v10:
>        1. Remove the "global values". @Parav Pandit
>        2. Avoid multiple interpretations of command behavior. @Alvaro Karsz
>
> v8->v9:
>        1. Declare the commands that can be sent for each feature. @Alvaro Karsz
>        2. Add information about "global values" in the command's explanation. @Alvaro Karsz
>
> v7->v8:
>        1. Use "best-effort" in Alvaro's patch instead of "the device may set the parameter to a value close to 2". @Michael S . Tsirkin, @David Edmondson
>
> v6->v7:
>        1. Clarify the relationship of VIRTIO_NET_CTRL_NOTF_COAL_TX/RX_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET. @Alvaro Karsz, @Michael S. Tsirkin
>        2. Remove the formula for vqn range. @Parav Pandit
>        3. Some expressions are clearer. @Parav Pandit, @Michael S. Tsirkin
>
> v5->v6:
>        1. Explain that the device may set a different value than the one passed in by the driver. @David Edmondson
>
> v4->v5:
>        1. Add the correspondence between virtio_net_ctrl_coal and virtio_net_ctrl_coal_vq and control commands. @Michael S. Tsirkin
>        2. Add read and write attributes for each field. @Michael S. Tsirkin
>        3. A clearer description of how to set coalescing parameters for vq reset. @Michael S. Tsirkin
>        4. Fix some syntax errors. @Michael S. Tsirkin, @David Edmondson
>
> v3->v4:
>        1. Include virtio_net_ctrl_coal in the virtio_net_ctrl_coal_vq structure. @Alvaro Karsz
>        2. Add consideration of vq reset. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
>        3. Avoid too many examples by giving a comprehensive example. @Michael S. Tsirkin
>        4. Fix typos and streamline clarifications. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
>
> v2->v3:
>        1. Add the netdim link. @Parav Pandit
>        2. VIRTIO_NET_F_VQ_NOTF_COAL no longer depends on VIRTIO_NET_F_NOTF_COAL. @Michael S. Tsirkin, @Alvaro Karsz
>        3. _VQ_GET is explained more. @Michael S. Tsirkin
>        4. Add more examples to avoid misunderstandings. @Michael S. Tsirkin
>        5. Clarify some statements. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
>        6. Adjust the virtio_net_ctrl_coal_vq structure. @Michael S. Tsirkin
>        7. Fix some typos. @Michael S. Tsirkin
>
> v1->v2:
>        1. Rename VIRTIO_NET_F_PERQUEUE_NOTF_COAL to VIRTIO_NET_F_VQ_NOTF_COAL. @Michael S. Tsirkin
>        2. Use the \field{vqn} instead of the qid. @Michael S. Tsirkin
>        3. Unify tx and rx control structures into one structure virtio_net_ctrl_coal_vq. @Michael S. Tsirkin
>        4. Add a new control command VIRTIO_NET_CTRL_NOTF_COAL_VQ. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
>        5. The special value 0xFFF is removed because VIRTIO_NET_CTRL_NOTF_COAL can be used. @Alvaro Karsz
>        6. Clarify some special scenarios. @Michael S. Tsirkin, @Parav Pandit, @Alvaro Karsz
>
>  device-types/net/description.tex | 88 ++++++++++++++++++++++++++++----
>  1 file changed, 77 insertions(+), 11 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index e71e33b..b5d45e8 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -83,6 +83,8 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
>      channel.
>  
> +\item[VIRTIO_NET_F_VQ_NOTF_COAL(52)] Device supports virtqueue notification coalescing.
> +
>  \item[VIRTIO_NET_F_NOTF_COAL(53)] Device supports notifications coalescing.
>  
>  \item[VIRTIO_NET_F_GUEST_USO4 (54)] Driver can receive USOv4 packets.
> @@ -139,6 +141,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_NOTF_COAL] Requires VIRTIO_NET_F_CTRL_VQ.
>  \item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
>  \item[VIRTIO_NET_F_RSS] Requires VIRTIO_NET_F_CTRL_VQ.
> +\item[VIRTIO_NET_F_VQ_NOTF_COAL] Requires VIRTIO_NET_F_CTRL_VQ.
>  \end{description}
>  
>  \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
> @@ -1505,13 +1508,13 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  
>  \paragraph{Notifications Coalescing}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
>  
> -If the VIRTIO_NET_F_NOTF_COAL feature is negotiated, the driver can
> -send control commands for dynamically changing the coalescing parameters.
> +If the VIRTIO_NET_F_NOTF_COAL feature is negotiated, a driver can send
> +commands VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET
> +for notification coalescing.

"to change the notification coalescing parameters."

>  
> -\begin{note}
> -The behavior of the device in response to these commands is best-effort:
> -the device may generate notifications more or less frequently than specified.
> -\end{note}
> +If the VIRTIO_NET_F_VQ_NOTF_COAL feature is negotiated, a driver can send
> +commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET
> +for notification coalescing.

"to change the per-virtqueue notification coalescing parameters."

>  
>  \begin{lstlisting}
>  struct virtio_net_ctrl_coal {
> @@ -1519,25 +1522,74 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>      le32 max_usecs;
>  };
>  
> +struct virtio_net_ctrl_coal_vq {
> +    le16 vqn;
> +    le16 reserved;
> +    struct virtio_net_ctrl_coal coal;
> +};
> +
>  #define VIRTIO_NET_CTRL_NOTF_COAL 6
>   #define VIRTIO_NET_CTRL_NOTF_COAL_TX_SET  0
>   #define VIRTIO_NET_CTRL_NOTF_COAL_RX_SET 1
> + #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET 2
> + #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET 3
>  \end{lstlisting}
>  
>  Coalescing parameters:
>  \begin{itemize}
> +\item \field{vqn}: The virtqueue number of an enabled transmit or receive virtqueue.
>  \item \field{max_usecs} for RX: Maximum number of microseconds to delay a RX notification.
>  \item \field{max_usecs} for TX: Maximum number of microseconds 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}
>  
> -The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
> +\field{reserved} is reserved and it is ignored by a device.
> +
> +Read/Write attributes for coalescing parameters:
> +\begin{itemize}
> +\item For commands VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET, \field{max_usecs}
> +      and \field{max_packets} are write-only for a driver.
> +\item For the command VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET, \field{vqn}, \field{reserved}, \field{max_usecs}
> +      and \field{max_packets} are write-only for a driver.
> +\item For the command VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET, \field{vqn} and \field{reserved} are write-only
> +      for a driver, and, \field{max_usecs} and \field{max_packets} are read-only for the driver.
> +\end{itemize}
> +
> +The class VIRTIO_NET_CTRL_NOTF_COAL has 4 commands:
>  \begin{enumerate}
> -\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{max_usecs} and \field{max_packets} parameters for all transmit virtqueues.
> -\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{max_usecs} and \field{max_packets} parameters for all receive virtqueues.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: use the structure virtio_net_ctrl_coal to set the \field{max_usecs} and \field{max_packets} parameters for all transmit virtqueues.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: use the structure virtio_net_ctrl_coal to set the \field{max_usecs} and \field{max_packets} parameters for all receive virtqueues.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET: use the structure virtio_net_ctrl_coal_vq to set the \field{max_usecs} and \field{max_packets} parameters
> +                                        for an enabled transmit/receive virtqueue whose number is \field{vqn}.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET: use the structure virtio_net_ctrl_coal_vq to get the \field{max_usecs} and \field{max_packets} parameters
> +                                        for an enabled transmit/receive virtqueue whose number is \field{vqn}.

Should this now be "whose index is \field{vqn}"?

>  \end{enumerate}
>  
> +\begin{note}
> +The behavior of the device in response to these commands is best-effort:
> +the device may generate notifications more or less frequently than specified.
> +\end{note}
> +
> +If coalescing parameters are being set, the device applies the last coalescing parameters received for a
> +virtqueue, regardless of the command used to set the parameters. Use the following command sequence
> +with 2 pairs of virtqueues as an example:
> +Each of the following commands sets \field{max_usecs} and \field{max_packets} parameters for virtqueues.
> +\begin{itemize}
> +\item Command1: VIRTIO_NET_CTRL_NOTF_COAL_RX_SET sets coalescing parameters for virtqueue0 and virtqueue2, and, virtqueue1 and virtqueue3 retain their previous parameter values.
> +\item Command2: VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET with \field{vqn} = 0 sets coalescing parameters for virtqueue0, and virtqueue2 retains the values from command1.
> +\item Command3: VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET with \field{vqn} = 0, the device responds with coalescing parameters of virtqueue0 set by command2.
> +\item Command4: VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET with \field{vqn} = 1 sets coalescing parameters for virtqueue1, and virtqueue3 retains its previous values.
> +\item Command5: VIRTIO_NET_CTRL_NOTF_COAL_TX_SET sets coalescing parameters for virtqueue1 and virtqueue3, and overrides the values set by command4.
> +\item Command6: VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET with \field{vqn} = 1, the device responds with coalescing parameters of virtqueue1 set by command5.
> +\end{itemize}
> +
> +Upon disabling and re-enabling the transmit virtqueue, the device will set the coalescing parameters of the virtqueue
> +to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, or to 0 if the command did not set any TX coalescing parameters.
> +
> +Upon disabling and re-enabling the receive virtqueue, the device will set the coalescing parameters of the virtqueue
> +to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command, or to 0 if the command did not set any RX coalescing parameters.
> +
>  \subparagraph{Operation}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / Operation}
>  
>  The device sends a used buffer notification once the notification conditions are met and if the notifications are not suppressed as explained in \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer Notification Suppression}.
> @@ -1585,11 +1637,25 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  
>  \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.
> +A driver MUST negotiate the VIRTIO_NET_F_NOTF_COAL feature before issuing commands VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRIO_NET_CTRL_NOTF_COAL_RX_SET.
> +
> +A driver MUST negotiate the VIRTIO_NET_F_VQ_NOTF_COAL feature before issuing commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRIO_NET_CTRL_NOTF_COAL_VQ_GET.
> +
> +A driver MUST ignore the values of coalescing parameters received from the VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command if a device responds with VIRTIO_NET_ERR.
>  
>  \devicenormative{\subparagraph}{Notifications Coalescing}{Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
>  
> -A device SHOULD respond to the VIRTIO_NET_CTRL_NOTF_COAL commands with VIRTIO_NET_ERR if it was not able to change the parameters.
> +A device MUST ignore \field{reserved}.
> +
> +A device SHOULD respond to VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands with VIRTIO_NET_ERR if it was not able to change the parameters.
> +
> +A device MUST respond to the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command with VIRTIO_NET_ERR if it was not able to change the parameters.
> +
> +A device MUST respond to VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET commands with VIRTIO_NET_ERR if the given virtqueue is disabled.
> +
> +After disabling and re-enabling a transmit/receive virtqueue, the device MUST set coalescing parameters of the virtqueue
> +to those configured using the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET/VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command, or, if the command
> +did not configure TX/RX coalescing parameters, to 0.
>  
>  A device SHOULD NOT send used buffer notifications to the driver if the notifications are suppressed, even if the notification conditions are met.
-- 
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