[PATCH v3] 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_USECS_SET:
  Ask the network device to change the rx-usecs and tx-usecs parameters.
  rx-usecs - Time to delay an RX notification after packet arrival in microseconds.
  tx-usecs - Time to delay a TX notification after a sending a packet in microseconds.

- VIRTIO_NET_CTRL_NOTF_COAL_FRAMES_SET:
  Ask the network device to change the rx-max-frames and tx-max-frames parameters.
  rx-max-frames - Number of packets to delay an RX notification after packet arrival.
  tx-max-frames - Number of packets to delay a TX notification after sending a packet.

--
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.

--

Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
---
 content.tex | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/content.tex b/content.tex
index 7508dd1..b0ee98d 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 +4467,71 @@ \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_usec {
+    le32 tx_usecs;
+    le32 rx_usecs;
+};
+
+struct virtio_net_ctrl_coal_frames {
+    le32 tx_frames_max;
+    le32 rx_frames_max;
+};
+
+#define VIRTIO_NET_CTRL_NOTF_COAL 6
+ #define VIRTIO_NET_CTRL_NOTF_COAL_USECS_SET  0
+ #define VIRTIO_NET_CTRL_NOTF_COAL_FRAMES_SET 1
+\end{lstlisting}
+
+The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
+\begin{itemize}
+\item VIRTIO_NET_CTRL_NOTF_COAL_USECS_SET: set the rx-usecs (time to delay an RX notification after frame arrival in microseconds) and tx-usecs (time to delay a TX notification after a sending a frame in microseconds) parameters.
+\item VIRTIO_NET_CTRL_NOTF_COAL_FRAMES_SET: set the rx-max-frames (number of frames to delay an RX notification after frame arrival) and tx-max-frames (number of frames to delay a TX notification after sending a frame) parameters.
+\end{itemize}
+
+\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_frames_max = 15.
+
+\begin{itemize}
+\item The device will count received frames until it accumulates 15 frames, or until 10 usecs elapsed since the arrival of the first frame.
+\item The device will check if at least one descriptor was used from the descriptor area, if not, it will continue to accumulate frames until one descriptor is used.\\
+An example is if any of the VIRTIO_NET_F_GUEST_TSO/UFO features are negotiated, a device could receive more than 15 frames, and write all in the same buffer.
+\item The device will reset its internal coalescing counters.
+\item The device will send a notification to the driver only if the driver hasn't suppressed notifications.
+\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_frames_max = 15.
+
+\begin{itemize}
+\item The device will count sent frames until it accumulates 15 frames, or until 10 usecs elapsed since first frame was sent.
+\item The device will check if at least one descriptor was used from the descriptor area, if not, it will continue to accumulate frames until one descriptor is used.\\
+An example is if any of the VIRTIO_NET_F_HOST_TSO/UFO features are negotiated, a device could receive a big buffer, which will take more than 15 frames to send.
+\item The device will reset its internal coalescing counters.
+\item The device will send a notification to the driver only if the driver hasn't suppressed notifications.
+\end{itemize}
+
+\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_frames_max/rx_frames_max values bigger than the queue size.\\ \\
+A device SHOULD NOT send notifications to the driver, if the driver asked to suppress notifications.\\ \\
+A device SHOULD initialize all coalescing values to 0. \\ \\
+
+
 \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
 Types / Network Device / Legacy Interface: Framing Requirements}
 
-- 
2.32.0