[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [PATCH v9 14/16] VIRTIO_F_NOTIFICATION_DATA: extra data to devices
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
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]