[virtio] [PATCH v9 14/16] VIRTIO_F_NOTIFICATION_DATA: extra data to devices

["Michael S. Tsirkin" <mst@redhat.com>]

Motivation for the new feature is included in the text.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 content.tex      | 141 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 introduction.tex |   4 +-
 notifications.c  |   3 ++
 3 files changed, 140 insertions(+), 8 deletions(-)
 create mode 100644 notifications.c

diff --git a/content.tex b/content.tex
index c57a918..4261913 100644
--- a/content.tex
+++ b/content.tex
@@ -283,9 +283,77 @@ Packed Virtqueues}).
 Every driver and device supports either the Packed or the Split
 Virtqueue format, or both.
 
+\subsection{Driver notifications} \label{sec:Virtqueues / Driver notifications}
+Driver is sometimes required to notify the device after
+making changes to the virtqueue.
+
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+this notification involves sending the
+virtqueue number to the device (depending on the transport).
+
+However, some devices benefit from ability to find out the number of
+available descriptors in the ring, and whether to send
+interrupts to drivers without accessing virtqueue in memory:
+for efficiency or as a debugging aid.
+
+To help with these optimizations, when VIRTIO_F_NOTIFICATION_DATA
+has been negotiated, driver notifications to the device include
+the following information:
+
+\begin{description}
+\item [VQ number]
+\item [Offset]
+      Within the ring where the next available ring entry
+      will be written.
+      Without VIRTIO_F_RING_PACKED this refers to the
+      15 least significant bits of the available index.
+      With VIRTIO_F_RING_PACKED this refers to the offset
+      (in units of descritor entries)
+      within the descriptor ring where the next available
+      descriptor will be written.
+\item [Wrap Counter]
+      With VIRTIO_F_RING_PACKED this is the wrap counter
+      referring to the next available descriptor.
+      Without VIRTIO_F_RING_PACKED this is the most significant bit
+      of the available index.
+\end{description}
+
+Note that driver can trigger multiple notifications even without
+making any more changes to the ring. When VIRTIO_F_NOTIFICATION_DATA
+has been negotiated, these notifications would then have
+identical \field{Offset} and \field{Wrap Counter} values.
+
 \input{split-ring.tex}
 
 \input{packed-ring.tex}
+
+\subsubsection{Driver notifications}
+
+\label{sec:Packed Virtqueues / Driver notifications}
+Whenever not suppressed by Device Event Suppression,
+driver is required to notify the device after
+making changes to the virtqueue.
+
+Some devices benefit from ability to find out the number of
+available descriptors in the ring, and whether to send
+interrupts to drivers without accessing virtqueue in memory:
+for efficiency or as a debugging aid.
+
+To help with these optimizations, driver notifications
+to the device include the following information:
+
+\begin{itemize}
+\item VQ number
+\item Offset (in units of descriptor size) within the ring
+      where the next available descriptor will be written
+\item Wrap Counter referring to the next available
+      descriptor
+\end{itemize}
+
+Note that driver can trigger multiple notifications even without
+making any more changes to the ring. These would then have
+identical \field{Offset} and \field{Wrap Counter} values.
+
 \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
 
 We start with an overview of device initialization, then expand on the
@@ -862,7 +930,9 @@ the same Queue Notify address for all queues.
 \devicenormative{\paragraph}{Notification capability}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
 The device MUST present at least one notification capability.
 
-The \field{cap.offset} MUST be 2-byte aligned.  
+For devices not offering VIRTIO_F_NOTIFICATION_DATA:
+
+The \field{cap.offset} MUST be 2-byte aligned.
 
 The device MUST either present \field{notify_off_multiplier} as an even power of 2,
 or present \field{notify_off_multiplier} as 0.
@@ -876,6 +946,23 @@ For all queues, the value \field{cap.length} presented by the device MUST satisf
 cap.length >= queue_notify_off * notify_off_multiplier + 2
 \end{lstlisting}
 
+For devices offering VIRTIO_F_NOTIFICATION_DATA:
+
+The device MUST either present \field{notify_off_multiplier} as a
+number that is a power of 2 that is also a multiple 4,
+or present \field{notify_off_multiplier} as 0.
+
+The \field{cap.offset} MUST be 4-byte aligned.
+
+The value \field{cap.length} presented by the device MUST be at least 4
+and MUST be large enough to support queue notification offsets
+for all supported queues in all possible configurations.
+
+For all queues, the value \field{cap.length} presented by the device MUST satisfy:
+\begin{lstlisting}
+cap.length >= queue_notify_off * notify_off_multiplier + 4
+\end{lstlisting}
+
 \subsubsection{ISR status capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / ISR status capability}
 
 The VIRTIO_PCI_CAP_ISR_CFG capability
@@ -1268,8 +1355,25 @@ separate cache lines.
 
 \subsubsection{Notifying The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Notifying The Device}
 
-The driver notifies the device by writing the 16-bit virtqueue index
-of this virtqueue to the Queue Notify address.  See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability} for how to calculate this address.
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+the driver notifies the device by writing the 16-bit virtqueue index
+of this virtqueue (in little-endian byte order format)
+to the Queue Notify address.
+
+When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+the driver notifies the device by writing the following
+32-bit value to the Queue Notify address:
+\begin{lstlisting}
+le32 vqn : 16,
+     next_off : 15,
+     next_wrap : 1;
+\end{lstlisting}
+
+See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}
+for the definition of the components.
+
+See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability} for how to calculate the
+Queue Notify address.
 
 \subsubsection{Virtqueue Interrupts From The Device}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Virtqueue Interrupts From The Device}
 
@@ -1500,8 +1604,19 @@ All register values are organized as Little Endian.
   }
   \hline 
   \mmioreg{QueueNotify}{Queue notifier}{0x050}{W}{%
-    Writing a queue index to this register notifies the device that
-    there are new buffers to process in the queue.
+    Writing a value this register notifies the device that
+    there are new buffers to process in a queue.
+
+    When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+    the value written is the queue index.
+
+    When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+    the value has the following format:
+
+    \lstinputlisting{notifications.c}
+
+    See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}
+    for the definition of the components.
   }
   \hline 
   \mmioreg{InterruptStatus}{Interrupt status}{0x60}{R}{%
@@ -2340,12 +2455,22 @@ GPR  &   Input Value     & Output Value \\
 \hline
   2   &  Subchannel ID    & Host Cookie  \\
 \hline
-  3   & Virtqueue number  &              \\
+  3   & Notification data &              \\
 \hline
   4   &   Host Cookie     &              \\
 \hline
 \end{tabular}
 
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+the \field{Notification data} includes the Virtqueue number.
+
+When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+the value has the following format:
+\lstinputlisting{notifications.c}
+
+See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}
+for the definition of the components.
+
 \devicenormative{\paragraph}{Guest->Host Notification}{Virtio Transport Options / Virtio over channel I/O / Device Operation / Guest->Host Notification}
 The device MUST ignore bits 0-31 (counting from the left) of GPR2.
 This aligns passing the subchannel ID with the way it is passed
@@ -5260,6 +5385,10 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
   \item[VIRTIO_F_IN_ORDER(35)] This feature indicates
   that all buffers are used by the device in the same
   order in which they have been made available.
+  \item[VIRTIO_F_NOTIFICATION_DATA(36)] This feature indicates
+  that drivers pass extra data (besides identifying the Virtqueue)
+  in their device notifications.
+  See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}.
 \end{description}
 
 \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
diff --git a/introduction.tex b/introduction.tex
index 3cb7a70..d0b770e 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -163,8 +163,8 @@ from the least significant to the most significant bit.
 
 For example:
 \begin{lstlisting}
-be16 A : 15;
-be16 B : 1;
+be16 A : 15,
+     B : 1;
 \end{lstlisting}
 documents the value A stored in the low 15 bit of a 16 bit
 integer and the value B stored in the high bit of the 16 bit
diff --git a/notifications.c b/notifications.c
new file mode 100644
index 0000000..2ae96d4
--- /dev/null
+++ b/notifications.c
@@ -0,0 +1,3 @@
+u32 vqn : 16,
+    next_off : 15,
+    next_wrap : 1;
-- 
MST


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