VIRTIO_F_NOTIFICATION_DATA: extra data to devices

Some devices benefit from ability to find out the number of available
descriptors in the ring: for efficiency or as a debugging aid.

To help with these optimizations, add a new feature:
VIRTIO_F_NOTIFICATION_DATA. When negotiated, driver notifications to the
device include this extra information.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/26
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

3 files changed, 124 insertions, 8 deletions

diff --git a/content.tex b/content.tex
index 282cb29..68b7db0 100644
--- a/content.tex
+++ b/content.tex
@@ -331,6 +331,46 @@ Virtqueue format, or both.
 \input{split-ring.tex}
 
 \input{packed-ring.tex}
+
+\subsection{Driver notifications} \label{sec:Virtqueues / Driver notifications}
+The driver is sometimes required to send an available buffer
+notification to the device.
+
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+this notification involves sending the
+virtqueue number to the device (method depending on the transport).
+
+However, some devices benefit from the ability to find out the
+amount of available data in the queue without accessing the 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 [vqn] VQ number to be notified.
+\item [next_off] Offset
+      within the ring where the next available ring entry
+      will be written.
+      When VIRTIO_F_RING_PACKED has not been negotiated this refers to the
+      15 least significant bits of the available index.
+      When VIRTIO_F_RING_PACKED has been negotiated this refers to the offset
+      (in units of descriptor entries)
+      within the descriptor ring where the next available
+      descriptor will be written.
+\item [next_wrap] 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
+      (bit 15) of the available index.
+\end{description}
+
+Note that the driver can send multiple notifications even without
+making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA
+has been negotiated, these notifications would then have
+identical \field{next_off} and \field{next_wrap} 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
@@ -909,7 +949,9 @@ 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.
@@ -923,6 +965,23 @@ 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
@@ -1315,9 +1374,21 @@ separate cache lines.
 
 \subsubsection{Available Buffer Notifications}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}
 
-The driver sends an available buffer notification to the device by writing
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+the driver sends an available buffer notification to 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.
+of this virtqueue to the Queue Notify address.
+
+When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+the driver sends an available buffer notification to the device by writing
+the following 32-bit value to the Queue Notify address:
+\lstinputlisting{notifications-le.c}
+
+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{Used Buffer Notifications}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Used Buffer Notifications}
 
@@ -1548,8 +1619,19 @@ 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 to 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 \field{Notification data} value has the following format:
+
+    \lstinputlisting{notifications-le.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}{%
@@ -1725,8 +1807,18 @@ The driver will typically initialize the virtual queue in the following way:
 
 \subsubsection{Available Buffer Notifications}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Available Buffer Notifications}
 
-The driver sends an available buffer notification to the device by
-writing the index of the queue to be notified to \field{QueueNotify}.
+When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
+the driver sends an available buffer notification to the device by writing
+the 16-bit virtqueue index
+of the queue to be notified to \field{QueueNotify}.
+
+When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+the driver sends an available buffer notification to the device by writing
+the following 32-bit value to \field{QueueNotify}:
+\lstinputlisting{notifications-le.c}
+
+See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}
+for the definition of the components.
 
 \subsubsection{Notifications From The Device}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Notifications From The Device}
 
@@ -2434,12 +2526,22 @@ 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} contains the Virtqueue number.
+
+When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
+the value has the following format:
+\lstinputlisting{notifications-be.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
@@ -5499,6 +5601,10 @@ Descriptors} and \ref{sec:Packed Virtqueues / Indirect Flag: Scatter-Gather Supp
   \item[VIRTIO_F_SR_IOV(37)] This feature indicates that
   the device supports Single Root I/O Virtualization.
   Currently only PCI devices support this feature.
+  \item[VIRTIO_F_NOTIFICATION_DATA(38)] This feature indicates
+  that the driver passes extra data (besides identifying the virtqueue)
+  in its 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/notifications-be.c b/notifications-be.c
new file mode 100644
index 0000000..5be947e
--- /dev/null
+++ b/notifications-be.c
@@ -0,0 +1,5 @@
+be32 {
+	vqn : 16;
+	next_off : 15;
+	next_wrap : 1;
+};
diff --git a/notifications-le.c b/notifications-le.c
new file mode 100644
index 0000000..fe51267
--- /dev/null
+++ b/notifications-le.c
@@ -0,0 +1,5 @@
+le32 {
+	vqn : 16;
+	next_off : 15;
+	next_wrap : 1;
+};