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Subject: Re: [virtio-comment] Re: [PATCH v7] virtio-net: support the virtqueue coalescing moderation
On Thursday, 2023-02-23 at 18:52:14 +08, Heng Qi wrote:
> Hi, David.
>
> å 2023/2/23 äå6:05, David Edmondson åé:
>> On Wednesday, 2023-02-22 at 22:06:32 +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 .
>>>
>>> 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 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
>> A couple of things about this:
>> - why say "a value close to a power of 2" - couldn't the device pick any
>> value it chooses?
>
> This is just a hint from the spec, it is "MAY", not "MUST" in the
> conformance of the device, the device can still set any value it
> receives.
Okay.
> And, since "virtqueue notification coalescing" feature will be used in
> the netdim [1] algorithm,
> and the coalescing moderation of netdim is roughly as follows, so it
> is allowed to give the hint in the spec:
> "
> #define NET_DIM_RX_EQE_PROFILES { \
> {.usec = 1, .pkts = NET_DIM_DEFAULT_RX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 8, .pkts = NET_DIM_DEFAULT_RX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 64, .pkts = NET_DIM_DEFAULT_RX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 128, .pkts = NET_DIM_DEFAULT_RX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 256, .pkts = NET_DIM_DEFAULT_RX_CQ_PKTS_FROM_EQE,} \
> }
>
> #define NET_DIM_RX_CQE_PROFILES { \
> {.usec = 2, .pkts = 256,}, \
> {.usec = 8, .pkts = 128,}, \
> {.usec = 16, .pkts = 64,}, \
> {.usec = 32, .pkts = 64,}, \
> {.usec = 64, .pkts = 64,} \
> }
>
> #define NET_DIM_TX_EQE_PROFILES { \
> {.usec = 1, .pkts = NET_DIM_DEFAULT_TX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 8, .pkts = NET_DIM_DEFAULT_TX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 32, .pkts = NET_DIM_DEFAULT_TX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 64, .pkts = NET_DIM_DEFAULT_TX_CQ_PKTS_FROM_EQE,}, \
> {.usec = 128, .pkts = NET_DIM_DEFAULT_TX_CQ_PKTS_FROM_EQE,} \
> }
>
> #define NET_DIM_TX_CQE_PROFILES { \
> {.usec = 5, .pkts = 128,}, \
> {.usec = 8, .pkts = 64,}, \
> {.usec = 16, .pkts = 32,}, \
> {.usec = 32, .pkts = 32,}, \
> {.usec = 64, .pkts = 32,} \
> }
> "
> [1]Â https://docs.kernel.org/networking/net_dim.html
>
>> - I think that we need to be more explicit that the values passed in the
>> SET request may not be honoured exactly.
>
> Yes, there are already examples in the current spec:
> "
> +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.
> "
> If you find this unclear, do you need more examples or clarification,
> or do you have a better way?
Explicit is good:
When a device receives a command of the VIRTIO_NET_CTRL_NOTF_COAL class
to set a coalescing parameter it may choose to use a value different to
that specified in the command, for example a power of two value close to
the specified parameter. The value chosen by the device can be retrieved
using the VIRTIO_NET_CTRL_NOTF_VQ_GET command.
>> - should the chosen value be returned in the SET call? (Not too fussed
>> about this, though it may result in an implementation immediately
>> calling GET after SET to see what actually happened.)
>
> As you said, I think we can just call GET to view.
>
>> - the example which shows how the global and per-VQ set operations
>> interact is reasonably worded ("the device responds with coalescing
>> parameters of virtqueue1 set by command5"), so that seems okay.
>
> Yeah.
>
> Thanks.
>
>>> 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 | 99 ++++++++++++++++++++++++++++++--
>>> 1 file changed, 94 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
>>> index e71e33b..745e4d9 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 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_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 of setting coalescing parameters as the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command repeated for
>>> + each virtqueue of transmitq1\ldots transmitqN.
>>> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: have the same effect of setting coalescing parameters 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 +1611,15 @@ \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 enabled after virtqueue reset, its coalescing parameters are set to global values.
>>> +
>>> \subparagraph{RX Example}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / RX Example}
>>> If, for example:
>>> @@ -1585,11 +1656,29 @@ \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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--
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