[virtio] Re: [PATCH v9 14/16] VIRTIO_F_NOTIFICATION_DATA: extra data to devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 07, 2018 at 12:11:58PM +0100, Cornelia Huck wrote:
> On Thu, 1 Mar 2018 01:31:37 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > Motivation for the new feature is included in the text.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  content.tex      | 141 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
> >  introduction.tex |   4 +-
> >  notifications.c  |   3 ++
> >  3 files changed, 140 insertions(+), 8 deletions(-)
> >  create mode 100644 notifications.c
> > 
> > diff --git a/content.tex b/content.tex
> > index c57a918..4261913 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -283,9 +283,77 @@ 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, and whether to send
> > +interrupts to drivers 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, and whether to send
> > +interrupts to drivers 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.
> 
> Isn't that duplicating the information for the generic case?
> 
> (And if you wanted to specify something specific for the packed case,
> shouldn't it go into packed-ring.tex?)

Right. Cut-n-pasted twice. Will drop.


> > +
> >  \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
> 
> (...)
> 
> > +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;
> 
> Don't we want to write this as
> 
> le32 vqn : 16;
> le32 next_off :15;
> le32 next_wrap : 1;
> 
> ?

Same thing in C, but would be more confusing IMHO since it will be up to
the reader to figure out which fields comprise the 32 bit integer.


> > +\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 +1604,19 @@ 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}
> 
> Doesn't mmio require this to be le explicitly?

Oops you are right. The Latex I guess I will have to share with PCI instead.

> > +
> > +    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 +2455,22 @@ 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}
> 
> And we probably want to make this be explicitly.

Are you sure?
I looked at s390 code and it just uses VQ index in native
endian-ness, so I kept this consistent.
No skin off my nose but pls let me know.

> > +
> > +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
> > @@ -5260,6 +5385,10 @@ 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}
> > diff --git a/introduction.tex b/introduction.tex
> > index 3cb7a70..d0b770e 100644
> > --- a/introduction.tex
> > +++ b/introduction.tex
> > @@ -163,8 +163,8 @@ from the least significant to the most significant bit.
> >  
> >  For example:
> >  \begin{lstlisting}
> > -be16 A : 15;
> > -be16 B : 1;
> > +be16 A : 15,
> > +     B : 1;
> 
> Why are you dropping the second be16?

To make it clearer these two fields are part of the same
integer, without need to count bits.

> >  \end{lstlisting}
> >  documents the value A stored in the low 15 bit of a 16 bit
> >  integer and the value B stored in the high bit of the 16 bit
> > diff --git a/notifications.c b/notifications.c
> > new file mode 100644
> > index 0000000..2ae96d4
> > --- /dev/null
> > +++ b/notifications.c
> > @@ -0,0 +1,3 @@
> > +u32 vqn : 16,
> > +    next_off : 15,
> > +    next_wrap : 1;
> 
> I'm wondering how useful the u32 notation is here.

It says vqn in low 16 bits of a 32 bit counter, so e.g.
on LE system byte 0 and on BE system byte 3.
Compare to 

be32 vqn : 16,
     next_off : 15,
     next_wrap : 1;

where we say it's in low 16 bits of a 32 bit BE integer,
so byte 3.

-- 
MST

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