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Subject: Re: [virtio-comment] [PATCH v6] virtio-net: support the virtqueue coalescing moderation
Hi Heng,
>
> 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 .
>
> 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 structres 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 | 100 +++++++++++++++++++++++++++++--
> 1 file changed, 95 insertions(+), 5 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index e71e33b..0d9f4b9 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}
> @@ -1508,6 +1511,14 @@ \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.
>
> +If the VIRTIO_NET_F_VQ_NOTF_COAL feature is negotiated:
> +\begin{itemize}
> +\item a driver can send a VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command to set coalescing parameters of a given
> + enabled transmit/receive virtqueue.
> +\item a driver can send a VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command to a device, and the device responds with
> + coalescing parameters of a given enabled transmit/receive virtqueue.
> +\end{itemize}
> +
> \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.
> @@ -1519,25 +1530,76 @@ \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}
>
> +The VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands use the
> +virtio_net_ctrl_coal structure to set \field{max_usecs} and \field{max_packets} for all
> +transmit/receive virtqueues.
> +
> +The VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command uses the virtio_net_ctrl_coal_vq structure
> +to set \field{max_usecs} and \field{max_packets} for the supplied virtqueue number \field{vqn}.
> +
> +The VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command gets the values of \field{max_usecs} and
> +\field{max_packets} of the specified virtqueue from the device by setting \field{vqn}
> +in the virtio_net_ctrl_coal_vq structure.
> +
> +# 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 a driver.
> +\end{itemize}
> +
> 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{vqn} points to an enabled transmit/receive virtqueue, and its value satisfies $ 0 \leq vqn < max_virtqueue_pairs \ast 2 $.
> +
> +\field{reserved} is reserved and it is ignored by a device.
> +
> +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_VQ_SET: 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: the device returns 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_TX_SET: have the same effect as the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command repeated for each virtqueue of transmitq1\ldots transmitqN.
You mention later that VIRTIO_NET_CTRL_NOTF_COAL_TX_SET sets the
global parameters and repeating VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET for
each transmitq doesn't, so it's not exactly the same effect.
Same for VIRTIO_NET_CTRL_NOTF_COAL_RX_SET.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: have the same effect as the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command repeated for each virtqueue of receiveq1\ldots receiveqN.
> \end{enumerate}
>
> +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. For example with 2 pairs of virtqueues:
> +# Command sequence
> +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}
> +
> \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}.
> @@ -1549,6 +1611,16 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>
> When the device has \field{max_usecs} = 0 or \field{max_packets} = 0, the notification conditions are met after every packet received/sent.
>
> +When a device receives a command of the VIRTIO_NET_CTRL_NOTF_COAL class to set a coalescing parameter,
> +it may set the parameter to a value close to a power of 2. For example:
> +If the device receives \field{max_usecs} = 7 from the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command, it may set \field{max_usecs} = 8 for a given enabled virtqueue.
> +
> +When the device receives the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands,
> +it saves the values of coalescing parameters as global values, and the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command
> +does not change the global values. If the device is reset, the global values will be set to 0.
> +
> +When a virtqueue is reset, its coalescing parameters are set to the global values.
> +
> \subparagraph{RX Example}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / RX Example}
>
> If, for example:
> @@ -1585,11 +1657,29 @@ \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.
> +If neither the VIRTIO_NET_F_NOTF_COAL nor the VIRTIO_NET_F_VQ_NOTF_COAL feature
> +has been negotiated, the driver MUST NOT issue VIRTIO_NET_CTRL_NOTF_COAL commands.
> +
> +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 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.
> +
> +The VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands set coalescing parameters for all transmit/receive
> +virtqueues respectively and values of coalescing parameters are recorded as global values by a device.
> +The device MUST set the global values of coalescing parameters to 0 after being reset.
> +The VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command sets the coalescing parameters for a given enabled virtqueue without changing the global values.
> +
> +After disabling and re-enabling a virtqueue, the device MUST revert coalescing parameters of the virtqueue to the global values.
> +
> +A device MAY set the coalescing parameter to a value close to a power of 2 value.
> +
> +A device MUST ignore \field{reserved}.
>
> 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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