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Subject: [PATCH v4] 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>
---
changes from v3:
- more wording changes suggested by Cornelia
changes from v2:
- wording changes suggested by Cornelia
changes from v1:
- rebased on master
- included support for MMIO
- minor wording fixes
content.tex | 122 ++++++++++++++++++++++++++++++++++++++++++---
notifications-be.c | 5 ++
notifications-le.c | 5 ++
3 files changed, 124 insertions(+), 8 deletions(-)
create mode 100644 notifications-be.c
create mode 100644 notifications-le.c
diff --git a/content.tex b/content.tex
index 5582c1c..633471e 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
@@ -5497,6 +5599,10 @@ 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;
+};
--
MST
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