[virtio-comment] [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[virtio-comment] [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Alvaro Karsz]

Control a network device notifications coalescing parameters using the control virtqueue.
A new control class was added: VIRTIO_NET_CTRL_NOTF_COAL.

This class provides 2 commands:
- VIRTIO_NET_CTRL_NOTF_COAL_TX_SET:
  Ask the network device to change the tx_usecs and tx_max_buffers parameters.
  - tx_usecs: Time to delay a notification after a TX buffer is used in microseconds.
  - tx_max_buffers: Number of TX buffers to delay a notification after a TX buffer is used.

- VIRTIO_NET_CTRL_NOTF_COAL_RX_SET:
  Ask the network device to change the rx_usecs and rx_max_buffers parameters.
  - rx_usecs: Time to delay a notification after a RX buffer is used in microseconds.
  - rx_max_buffers: Number of RX buffers to delay a notification after a RX buffer is used.
--
v2:
	- Usage of notification terminology.
	- Add a few lines on what device should do when driver asked to
	  suppress notifications.

v3:
	- Remove whitespaces.
	- Explain with examples how the device should act.

v4:
	- Example of a scenarion when VIRTIO_F_EVENT_IDX is negotiated.
	- Usage of separate commands for RX coalescing and TX  coalescing.
--

Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
---
 content.tex | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/content.tex b/content.tex
index 7508dd1..2c75056 100644
--- a/content.tex
+++ b/content.tex
@@ -3084,6 +3084,8 @@ \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_NOTF_COAL(55)] Device supports notifications coalescing.
+
 \item[VIRTIO_NET_F_HOST_USO (56)] Device can receive USO packets. Unlike UFO
  (fragmenting the packet) the USO splits large UDP packet
  to several segments when each of these smaller packets has UDP header.
@@ -3129,6 +3131,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
 \item[VIRTIO_NET_F_GUEST_ANNOUNCE] Requires VIRTIO_NET_F_CTRL_VQ.
 \item[VIRTIO_NET_F_MQ] Requires VIRTIO_NET_F_CTRL_VQ.
 \item[VIRTIO_NET_F_CTRL_MAC_ADDR] Requires VIRTIO_NET_F_CTRL_VQ.
+\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.
 \end{description}
@@ -4464,6 +4467,91 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 (necessarily when not using the legacy interface) little-endian.
 
 
+\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.
+
+\begin{lstlisting}
+struct virtio_net_ctrl_coal_rx {
+    le32 rx_max_buffers;
+    le32 rx_usecs;
+};
+
+struct virtio_net_ctrl_coal_tx {
+    le32 tx_max_buffers;
+    le32 tx_usecs;
+};
+
+#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
+\end{lstlisting}
+
+Coalescing parameters:
+\begin{itemize}
+\item rx_usecs: Time to delay a notification after a RX buffer is used in microseconds.
+\item tx_usecs: Time to delay a notification after a TX buffer is used in microseconds.
+\item rx_max_buffers: Number of RX buffers to delay a notification after a RX buffer is used.
+\item tx_max_buffers: Number of TX buffers to delay a notification after a TX buffer is used.
+\end{itemize}
+
+Note: A chain of buffers counts as one, just like it would if this feature is not negotiated.
+
+The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
+\begin{itemize}
+\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the tx_usecs and tx_max_buffers parameters.
+\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the rx_usecs and rx_max_buffers parameters.
+\end{itemize}
+
+Note: rx_usecs = 0 or rx_max_buffers = 0 means that a notification should be sent for every used buffer, so if one of these values is 0, the other value is meaningless.
+The same is true for tx_usecs and tx_max_buffers.
+
+\paragraph{RX Notifications}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / RX Notifications}
+
+If, for example, rx_usecs = 10 and rx_buffers_max = 15.
+
+\begin{itemize}
+\item The device will count used buffers from the RX virtqueue until it accumulates 15, or until 10 usecs elapsed since the usage of the first one.
+\item The device will send a notification to the driver, only if the driver hasn't suppressed notifications.
+\item The device will reset its internal coalescing counters, if a notification was sent.
+\end{itemize}
+
+\paragraph{TX Notifications}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / TX Notifications}
+
+If, for example, tx_usecs = 10 and tx_buffers_max = 15.
+
+\begin{itemize}
+\item The device will count used buffers from the TX virtqueue until it accumulates 15, or until 10 usecs elapsed since the usage of the first one.
+\item The device will send a notification to the driver, only if the driver hasn't suppressed notifications.
+\item The device will reset its internal coalescing counters, if a notification was sent.
+\end{itemize}
+
+\paragraph{Coalescing and Notifications Suppression}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / Coalescing and Notifications Suppression}
+
+If the driver sets the VIRTQ_AVAIL_F_NO_INTERRUPT flag, the device should not send notifications to the driver. \\
+In this case, the device will increase the coalescing counters until the flag is cleared, and only then the device will send a notification and clear the coalescing counters. \\ \\
+
+If the VIRTIO_F_EVENT_IDX feature is negotiated, and used_event field is set, the device will send a notification only after used_event index is reached, even if the coalescing counters expired before.
+
+For example: \\
+rx_usecs = 10, rx_max_buffers = 7, used_event = 5, and current virtqueue location is 0: \\
+If the timer elapses before the \#5 buffer is used, the device will send a notification only after the \#5 buffer is used. \\
+If the \#5 buffer is used before the timer elapses, the device will send a notification after the \#7 buffer is used, or after the timer elapses, whichever occurs first.
+
+\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.
+
+\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 was not able to change the parameters.\\ \\
+A device MUST NOT accept tx_buffers_max/rx_buffers_max values bigger than the virtqueue size.\\ \\
+A device SHOULD NOT send notifications to the driver, if the notifications are suppressed.\\ \\
+A device SHOULD initialize all coalescing values to 0, meaning that a notification is sent after every used buffer (if events aren't suppressed). \\ \\
+
+
 \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
 Types / Network Device / Legacy Interface: Framing Requirements}
 
-- 
2.32.0


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Re: [virtio-comment] [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Cornelia Huck]

On Wed, Jun 01 2022, Alvaro Karsz <alvaro.karsz@solid-run.com> wrote:

> Control a network device notifications coalescing parameters using the control virtqueue.
> A new control class was added: VIRTIO_NET_CTRL_NOTF_COAL.
>
> This class provides 2 commands:
> - VIRTIO_NET_CTRL_NOTF_COAL_TX_SET:
>   Ask the network device to change the tx_usecs and tx_max_buffers parameters.
>   - tx_usecs: Time to delay a notification after a TX buffer is used in microseconds.
>   - tx_max_buffers: Number of TX buffers to delay a notification after a TX buffer is used.
>
> - VIRTIO_NET_CTRL_NOTF_COAL_RX_SET:
>   Ask the network device to change the rx_usecs and rx_max_buffers parameters.
>   - rx_usecs: Time to delay a notification after a RX buffer is used in microseconds.
>   - rx_max_buffers: Number of RX buffers to delay a notification after a RX buffer is used.
> --
> v2:
> 	- Usage of notification terminology.
> 	- Add a few lines on what device should do when driver asked to
> 	  suppress notifications.
>
> v3:
> 	- Remove whitespaces.
> 	- Explain with examples how the device should act.
>
> v4:
> 	- Example of a scenarion when VIRTIO_F_EVENT_IDX is negotiated.
> 	- Usage of separate commands for RX coalescing and TX  coalescing.
> --
>
> Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
> ---
>  content.tex | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
>

(...)

> +\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.
> +
> +\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 was not able to change the parameters.\\ \\
> +A device MUST NOT accept tx_buffers_max/rx_buffers_max values bigger than the virtqueue size.\\ \\
> +A device SHOULD NOT send notifications to the driver, if the notifications are suppressed.\\ \\
> +A device SHOULD initialize all coalescing values to 0, meaning that a notification is sent after every used buffer (if events aren't suppressed). \\ \\
> +
> +

I think these are missing being referenced in conformance.tex?

[This seems to be easy to miss, I also noticed missing statements while
preparing the changelong for 1.2. Should the build process maybe check
that conformance sections are hooked up properly?]


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Re: [virtio-comment] [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Alvaro Karsz]

> I think these are missing being referenced in conformance.tex?

Sorry, I forgot about conformance.tex.
I'll create a new version next week.

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[virtio-comment] Re: [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Jason Wang]

On Wed, Jun 1, 2022 at 3:33 PM Alvaro Karsz <alvaro.karsz@solid-run.com> wrote:
>
> Control a network device notifications coalescing parameters using the control virtqueue.
> A new control class was added: VIRTIO_NET_CTRL_NOTF_COAL.
>
> This class provides 2 commands:
> - VIRTIO_NET_CTRL_NOTF_COAL_TX_SET:
>   Ask the network device to change the tx_usecs and tx_max_buffers parameters.
>   - tx_usecs: Time to delay a notification after a TX buffer is used in microseconds.
>   - tx_max_buffers: Number of TX buffers to delay a notification after a TX buffer is used.
>
> - VIRTIO_NET_CTRL_NOTF_COAL_RX_SET:
>   Ask the network device to change the rx_usecs and rx_max_buffers parameters.
>   - rx_usecs: Time to delay a notification after a RX buffer is used in microseconds.
>   - rx_max_buffers: Number of RX buffers to delay a notification after a RX buffer is used.
> --
> v2:
>         - Usage of notification terminology.
>         - Add a few lines on what device should do when driver asked to
>           suppress notifications.
>
> v3:
>         - Remove whitespaces.
>         - Explain with examples how the device should act.
>
> v4:
>         - Example of a scenarion when VIRTIO_F_EVENT_IDX is negotiated.
>         - Usage of separate commands for RX coalescing and TX  coalescing.
> --
>
> Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
> ---
>  content.tex | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
>
> diff --git a/content.tex b/content.tex
> index 7508dd1..2c75056 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3084,6 +3084,8 @@ \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_NOTF_COAL(55)] Device supports notifications coalescing.
> +
>  \item[VIRTIO_NET_F_HOST_USO (56)] Device can receive USO packets. Unlike UFO
>   (fragmenting the packet) the USO splits large UDP packet
>   to several segments when each of these smaller packets has UDP header.
> @@ -3129,6 +3131,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_GUEST_ANNOUNCE] Requires VIRTIO_NET_F_CTRL_VQ.
>  \item[VIRTIO_NET_F_MQ] Requires VIRTIO_NET_F_CTRL_VQ.
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR] Requires VIRTIO_NET_F_CTRL_VQ.
> +\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.
>  \end{description}
> @@ -4464,6 +4467,91 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  (necessarily when not using the legacy interface) little-endian.
>
>
> +\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.
> +
> +\begin{lstlisting}
> +struct virtio_net_ctrl_coal_rx {
> +    le32 rx_max_buffers;
> +    le32 rx_usecs;
> +};
> +
> +struct virtio_net_ctrl_coal_tx {
> +    le32 tx_max_buffers;

Considering RX may use multiple buffers per packet, I wonder if it's
better to stick to "packet" here.

> +    le32 tx_usecs;
> +};
> +
> +#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
> +\end{lstlisting}
> +
> +Coalescing parameters:
> +\begin{itemize}
> +\item rx_usecs: Time to delay a notification after a RX buffer is used in microseconds.
> +\item tx_usecs: Time to delay a notification after a TX buffer is used in microseconds.
> +\item rx_max_buffers: Number of RX buffers to delay a notification after a RX buffer is used.

Does this work like: if we set rx_max_buffers to 10, it means after 1
buffers is used, we still need another 10 buffers?

> +\item tx_max_buffers: Number of TX buffers to delay a notification after a TX buffer is used.
> +\end{itemize}
> +
> +Note: A chain of buffers counts as one, just like it would if this feature is not negotiated.

I think we should explicitly say mergeable rx buffers here. And that's
why I think mentioning "packet is sent/received" is probably better
than the buffer used here.

> +
> +The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
> +\begin{itemize}
> +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the tx_usecs and tx_max_buffers parameters.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the rx_usecs and rx_max_buffers parameters.
> +\end{itemize}
> +
> +Note: rx_usecs = 0 or rx_max_buffers = 0 means that a notification should be sent for every used buffer, so if one of these values is 0, the other value is meaningless.
> +The same is true for tx_usecs and tx_max_buffers.

It's better to clarify it in the part of coalescing parameters, or we
can just borrow the comments of kernel's ethtool_coalesce, then it
should explain itself like:

@rx_usecs: How many usecs to delay an RX notification after a packet arrives
@rx_max_buffers: Maximum number of packets to receive before an RX notification

Not a native speaker but then rx_usecs=0 it means the notification is
sent immediately and by saying "maximum number" it means best effort.

> +
> +\paragraph{RX Notifications}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / RX Notifications}
> +
> +If, for example, rx_usecs = 10 and rx_buffers_max = 15.
> +
> +\begin{itemize}
> +\item The device will count used buffers from the RX virtqueue until it accumulates 15, or until 10 usecs elapsed since the usage of the first one.
> +\item The device will send a notification to the driver, only if the driver hasn't suppressed notifications.

Let's clarify what happens if notification is suppressed.

> +\item The device will reset its internal coalescing counters, if a notification was sent.

I think there's probably no need to mention the implementation details
like counters here. The definition of rx_usecs and rx_max_buffers
should be sufficient.

> +\end{itemize}
> +
> +\paragraph{TX Notifications}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / TX Notifications}
> +
> +If, for example, tx_usecs = 10 and tx_buffers_max = 15.
> +
> +\begin{itemize}
> +\item The device will count used buffers from the TX virtqueue until it accumulates 15, or until 10 usecs elapsed since the usage of the first one.
> +\item The device will send a notification to the driver, only if the driver hasn't suppressed notifications.
> +\item The device will reset its internal coalescing counters, if a notification was sent.
> +\end{itemize}
> +
> +\paragraph{Coalescing and Notifications Suppression}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing / Coalescing and Notifications Suppression}
> +
> +If the driver sets the VIRTQ_AVAIL_F_NO_INTERRUPT flag, the device should not send notifications to the driver. \\
> +In this case, the device will increase the coalescing counters until the flag is cleared, and only then the device will send a notification and clear the coalescing counters. \\ \\
> +
> +If the VIRTIO_F_EVENT_IDX feature is negotiated, and used_event field is set, the device will send a notification only after used_event index is reached, even if the coalescing counters expired before.
> +
> +For example: \\
> +rx_usecs = 10, rx_max_buffers = 7, used_event = 5, and current virtqueue location is 0: \\
> +If the timer elapses before the \#5 buffer is used, the device will send a notification only after the \#5 buffer is used. \\
> +If the \#5 buffer is used before the timer elapses, the device will send a notification after the \#7 buffer is used, or after the timer elapses, whichever occurs first.
> +
> +\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.
> +
> +\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 was not able to change the parameters.\\ \\
> +A device MUST NOT accept tx_buffers_max/rx_buffers_max values bigger than the virtqueue size.\\ \\

What's the reason for this?

> +A device SHOULD NOT send notifications to the driver, if the notifications are suppressed.\\ \\
> +A device SHOULD initialize all coalescing values to 0, meaning that a notification is sent after every used buffer (if events aren't suppressed). \\ \\

I think we need use MUST here, something like:

Upon reset, the device must present all zero values ...

Thanks

> +
> +
>  \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
>  Types / Network Device / Legacy Interface: Framing Requirements}
>
> --
> 2.32.0
>


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[virtio-comment] Re: [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Alvaro Karsz]

> Considering RX may use multiple buffers per packet, I wonder if it's
> better to stick to "packet" here.

Noted.

> Does this work like: if we set rx_max_buffers to 10, it means after 1
> buffers is used, we still need another 10 buffers?

No, if  rx_max_buffers=10, we should send a notification on the #10
packet. I will clarify that.

> I think we should explicitly say mergeable rx buffers here. And that's
> why I think mentioning "packet is sent/received" is probably better
> than the buffer used here.

I meant when the host writes a packet in several buffers, and the NEXT
flag is set on the descriptor.
But you're right, I should mention the mergeable rx buffers as well.

> It's better to clarify it in the part of coalescing parameters, or we
> can just borrow the comments of kernel's ethtool_coalesce, then it
> should explain itself like:
>
>
> @rx_usecs: How many usecs to delay an RX notification after a packet arrives
> @rx_max_buffers: Maximum number of packets to receive before an RX notification
>
>
> Not a native speaker but then rx_usecs=0 it means the notification is
> sent immediately and by saying "maximum number" it means best effort.

Noted. I will move the coalescing parameters explanation to this subparagraph.

> Let's clarify what happens if notification is suppressed.

I will add that in this case, the device should not send a notification.

> I think there's probably no need to mention the implementation details
> like counters here. The definition of rx_usecs and rx_max_buffers
> should be sufficient.

Noted.

> What's the reason for this?

Because if (tx/rx)_buffers_max is bigger than the virtqueue size, this
condition will never be met.
Do you think that we should allow this to happen?

> I think we need use MUST here, something like:
>
>
> Upon reset, the device must present all zero values ...

Noted.

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[virtio-comment] Re: [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Jason Wang]

On Thu, Jun 9, 2022 at 3:17 PM Alvaro Karsz <alvaro.karsz@solid-run.com> wrote:
>
> > Considering RX may use multiple buffers per packet, I wonder if it's
> > better to stick to "packet" here.
>
> Noted.
>
> > Does this work like: if we set rx_max_buffers to 10, it means after 1
> > buffers is used, we still need another 10 buffers?
>
> No, if  rx_max_buffers=10, we should send a notification on the #10
> packet. I will clarify that.
>
> > I think we should explicitly say mergeable rx buffers here. And that's
> > why I think mentioning "packet is sent/received" is probably better
> > than the buffer used here.
>
> I meant when the host writes a packet in several buffers, and the NEXT
> flag is set on the descriptor.
> But you're right, I should mention the mergeable rx buffers as well.
>
> > It's better to clarify it in the part of coalescing parameters, or we
> > can just borrow the comments of kernel's ethtool_coalesce, then it
> > should explain itself like:
> >
> >
> > @rx_usecs: How many usecs to delay an RX notification after a packet arrives
> > @rx_max_buffers: Maximum number of packets to receive before an RX notification
> >
> >
> > Not a native speaker but then rx_usecs=0 it means the notification is
> > sent immediately and by saying "maximum number" it means best effort.
>
> Noted. I will move the coalescing parameters explanation to this subparagraph.
>
> > Let's clarify what happens if notification is suppressed.
>
> I will add that in this case, the device should not send a notification.

Yes, and when the notification needs to be sent.

>
> > I think there's probably no need to mention the implementation details
> > like counters here. The definition of rx_usecs and rx_max_buffers
> > should be sufficient.
>
> Noted.
>
> > What's the reason for this?
>
> Because if (tx/rx)_buffers_max is bigger than the virtqueue size, this
> condition will never be met.

I don't understand here, the driver can poll for the used buffer actually.

> Do you think that we should allow this to happen?

Not sure but I just have a try with mlx4, it gives me:

# ethtool -g ens3d1
Ring parameters for ens3d1:
Pre-set maximums:
RX: 8192
RX Mini: n/a
RX Jumbo: n/a
TX: 8192
Current hardware settings:
RX: 1024
RX Mini: n/a
RX Jumbo: n/a
TX: 1024

# ethtool -c ens3d1
Coalesce parameters for ens3d1:
Adaptive RX: on  TX: n/a
stats-block-usecs: n/a
sample-interval: 0
pkt-rate-low: 400000
pkt-rate-high: 450000

rx-usecs: 16
rx-frames: 2048
rx-usecs-irq: n/a
rx-frames-irq: n/a

tx-usecs: 16
tx-frames: 16
tx-usecs-irq: n/a
tx-frames-irq: 256

rx-usecs-low: 0
rx-frame-low: n/a
tx-usecs-low: n/a
tx-frame-low: n/a

rx-usecs-high: 128
rx-frame-high: n/a
tx-usecs-high: n/a
tx-frame-high: n/a

It means rx-frames could be bigger than RX.

Thanks

>
> > I think we need use MUST here, something like:
> >
> >
> > Upon reset, the device must present all zero values ...
>
> Noted.
>


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[virtio-comment] Re: [PATCH v4] Introduction of Virtio Network device notifications coalescing feature.

[Alvaro Karsz]

> It means rx-frames could be bigger than RX.

Noted.
I will include all the changes in the new version.

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