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Diffstat (limited to 'content.tex')
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1 files changed, 103 insertions, 6 deletions
diff --git a/content.tex b/content.tex index 7a92cb1..ae5fa4c 100644 --- a/content.tex +++ b/content.tex @@ -283,9 +283,49 @@ 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} +The 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 (method depending on the transport). + +However, some devices benefit from the ability to find out the number of +available descriptors in the ring 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 [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 descriptor 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 the 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} + \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 +902,9 @@ 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 +918,23 @@ 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 +1327,21 @@ 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: +\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{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 +1572,19 @@ 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-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}{% @@ -2340,12 +2423,22 @@ 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 @@ -5348,6 +5441,10 @@ 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} |