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

[Alvaro Karsz <alvaro.karsz@solid-run.com>]

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