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Subject: Re: [PATCH v9] virtio-net: support the virtqueue coalescing moderation
Hi,
> 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 .
>
> 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 | 95 +++++++++++++++++++++++++++++---
> 1 file changed, 87 insertions(+), 8 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index e71e33b..71f6827 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}
> @@ -1505,8 +1508,17 @@ \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, the driver can send the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET
> +and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands to dynamically change the coalescing parameters for all transmit
> +and receive virtqueues, respectively.
> +
> +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}
>
The VQ_NOTF_COAL commands are enclosed in an itemize and the NOTF_COAL
commands aren't.
I think that we should be consistent here.
I'm not sure that describing the commands in here is necessary, maybe
just mentioning which commands can be used with which feature bit is
enough (this is what I meant in v8, sorry if I wasn't clear).
I'm not saying that describing the commands in here is not good, but
notice that the commands are described in 3 different places.
This is #1.
> \begin{note}
> The behavior of the device in response to these commands is best-effort:
> @@ -1519,25 +1531,76 @@ \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.
> +
This is #2.
> +# 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}
> +
> 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.
> +
> +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: set the \field{max_usecs} and \field{max_packets} parameters for all transmit virtqueues, and values of
> + coalescing parameters are recorded as TX global values by a device.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{max_usecs} and \field{max_packets} parameters for all receive virtqueues, and values of
> + coalescing parameters are recorded as RX global values by a device.
> +\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}, and do not change the TX/RX global values.
> +\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}.
> \end{enumerate}
This is #3.
>
> +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 +1612,11 @@ \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 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 enabled after virtqueue reset, its coalescing parameters are set to global values.
> +
Maybe this explanation should be put closer to the commands
descriptions, where the global coalescing parameters are mentioned for
the first time.
Something like:
....
\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}.
The device maintains global coalescing parameters for TX and RX....
And maybe we should use "global coalescing parameters" instead of
"global values" (in devicenormative as well).
> \subparagraph{RX Example}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / RX Example}
>
> If, for example:
> @@ -1585,15 +1653,26 @@ \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.
Maybe this part can be splitted to:
if the F1 feature has not been negotiated, the driver must not issue
commands C1,C2.
if the F2 feature has not been negotiated, the driver must not issue
commands C3,C4.
> +
> +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.
> +
> +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.
>
> -Upon reset, a device MUST initialize all coalescing parameters to 0.
> +After disabling and re-enabling a virtqueue, the device MUST revert coalescing parameters of the virtqueue to the global values.
> +
> +Upon reset, a device MUST initialize all coalescing parameters to 0 and MUST set the global values to 0.
>
> \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
> Types / Network Device / Legacy Interface: Framing Requirements}
> --
Let's wait for more comments before the next version, I guess some may
not agree with my comments.
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