On Tue, 27 Mar 2018 05:59:27 +0300
"Michael S. Tsirkin" <m...@redhat.com> wrote:
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.
Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---
v10 -> v11:
drop mention of interrupts: current proposal does not include
this interrupt related information
drop unrelated introduction sections
content.tex | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 133 insertions(+), 6 deletions(-)
diff --git a/content.tex b/content.tex
index 7a92cb1..95e9b82 100644
--- a/content.tex
+++ b/content.tex
@@ -283,9 +283,75 @@ 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 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 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 +928,9 @@ 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 +944,23 @@ 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 +1353,25 @@ 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 +1602,19 @@ 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 +2453,22 @@ 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
@@ -5348,6 +5471,10 @@ 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}
