[virtio] Re: [PATCH v11] VIRTIO_F_NOTIFICATION_DATA: extra data to devices

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 27 Mar 2018 05:59:27 +0300
"Michael S. Tsirkin" <mst@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 <mst@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

s/Driver/The driver/

> +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).

"method depending on the transport"?

> +
> +However, some devices benefit from ability to find out the number of

s/ability/the ability/

> +available descriptors in the ring without accessing virtqueue in memory:

s/virtqueue/the virtqueue/

> +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)

s/descritor/descriptor/

> +      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

s/driver/the driver/

> +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.

Shouldn't you drop this subsubsection? It just repeats what you stated
above, with less details.

> +
>  \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}

Don't you need to write this down explicitly, as in the pci case?

> +
> +    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.

s/includes/contains/

> +
> +When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
> +the value has the following format:
> +\lstinputlisting{notifications.c}

This also needs to be spelled out explicitly (in big endian format).

> +
> +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}


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