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Subject: Re: [virtio-comment] Re: [PATCH v7] virtio-net: support the virtqueue coalescing moderation
On Thu, Feb 23, 2023 at 06:52:14PM +0800, 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.
I think a power of 2 is an example that confuses more than it clarifies.
> 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?
>
> > - 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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