Re: [virtio] [PATCH v12] VIRTIO_F_NOTIFICATION_DATA: extra data to devices

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 28 Mar 2018 20:21:47 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Mar 28, 2018 at 06:56:13PM +0200, Halil Pasic wrote:
> > 
> > 
> > On 03/27/2018 06:37 PM, Michael S. Tsirkin 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.  
> > 
> > I'm pondering about symmetry and about the nature of these
> > optimizations. By symmetry I mean this only works driver->device.
> > Why do we want this the one but not the other way around?  
> 
> Because
> - memory is already closer to CPU than to devices
> - there is no way to pass extra data with an interrupt
> 
> 
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > > 
> > > v11 -> v12
> > > 	add missing 'the' article
> > > 	drop duplicate text sections
> > > 	use separate listing files for BE and LE, making
> > > 	format explicit.
> > > 
> > > v10 -> v11:
> > >         drop mention of interrupts: current proposal does not include
> > >         this interrupt related information
> > >         drop unrelated introduction sections
> > > 
> > > 
> > > 
> > >  content.tex        | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++---
> > >  notifications-be.c |   5 +++
> > >  notifications-le.c |   5 +++
> > >  3 files changed, 113 insertions(+), 6 deletions(-)
> > >  create mode 100644 notifications-be.c
> > >  create mode 100644 notifications-le.c
> > > 
> > > 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}  
> > 
> > Don't we call the same 'device notification' in 2.5.12.4 Notifying The Device.  
> 
> Oh right.
> 
> > The wordings used to denote the different notifications seems to be
> > chaotic and maybe somewhat confusing overall.
> > 
> > Let me provide some examples:
> > For *virtqueue notification sent by the driver to the device* we use:
> >    * 'driver notifications' (here)  
> 
> We should fix this I think.
> 
> >    * 'device notification' in 2.5.12.4 Notifying The Device)
> >    * 'virtqueue notification' (2.5.9 Virtqueue Notification Suppression)  
> 
> Let's change these to 'virtqueue device notifications'?
> It's a separate issue though, not introduced by this patch.
> 
> >    * 'guest -> host notification' (4.3.3.2 Guest->Host Notification)  
> 
> That's a transport thing specific to virtio over MMIO.

ccw, right?

I called these 'guest->host' (and 'host->guest') because it was more
easily understandable at the time. It would be better to call them
'device notification' and 'driver notification' to align with the
wording elsewhere. (As a separate patch.)

> 
> > The notifications *sent by the device to the driver (virtqueue or
> > configuration change)* are often referred to as *interrupts* but occasionally
> > also as notifications (e.g. 'host->guest notification').

I think 'notifications' is a better term for these. I'm thinking of a
driver polling for outstanding notifications from the device, no
interrupts involved.

But we can discuss that later.

> > 
> > 
> >   
> > > +The driver is sometimes required to notify the device after
> > > +making changes to the virtqueue.
> > > +  
> > 
> > I don't like sometimes. I would prefer something like
> > 'Under certain circumstances the driver is required issue
> > a virtqueue notification to the device.  
> 
> 
> Under certain circumstances just says sometimes in 3 words.
> Can't we find a way to say it that does not bump the word count x3?

What's wrong with "Sometimes, the driver is required to issue..."?

> 
> > The method is transport
> > defined.'  
> 
> That's good to add.

Agreed.

> 
> > > +When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
> > > +this notification involves sending the
> > > +virtqueue number to the device (method depending on the transport).  
> > 
> > I don't like 'involves' here. I think it's supposed to tell
> > us that notification is a vitqueue level operation. Moreover
> > the point is  when VIRTIO_F_NOTIFICATION_DATA has not been negotiated
> > it's a virtueue notification and nothing more.

What about

"When VIRTIO_F_NOTIFICATION_DATA has not been negotiated, the driver
notifies the device about the affected virtqueue only."

?

> >   
> > > +
> > > +However, some devices benefit from the ability to find out the number of
> > > +available descriptors in the ring without accessing the virtqueue in memory:  
> > 
> > What is 'the ring'? (probably descriptor ring or available ring depending
> > on what do we have)  
> 
> Maybe buffers in the virtqueue?

Works for me.

> 
> > > +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)  
> > 
> > What does '(in units of descriptor entries)' mean?  
> 
> That it's not an address in bytes.
> Offset is a distance right?
> Within ring implies from start of ring.
> What is left is to tell what are the units.
> 
> > Is it a
> > mod ring size index?  
> 
> I don't see how specifying units can be taken
> to mean it's modulo some value :(
> 
> > If it is, why not call it index consequently
> > (instead of this offset-index-offset thing)?  
> 
> In fact index in many places is *not* mod ring size.
> For me offset means where it is. Just need to specify
> the units.

I agree, and I think the wording is fine here.

> 
> 
> > > +      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)  
> > 
> > s/drivers/driver/ plural seems inappropriate
> >   
> > > +  in their device notifications.  

As a feature is negotiated per-device anyway, agreed.

"This feature indicates that the driver passes extra data (besides
identifying the virtqueue) in its device notifications."

> > 
> > I'm not really satisfied with this description but have nothing
> > better to offer, so I'm willing to accept.
> > 
> >   
> > > +  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;  
> > 
> > Would it make sense to switch next_wrap and next_off for be.
> > I mean that way we should be able to read the avail_idx at
> > once (I think) instead of having to compute it first from
> > next_off and next_wrap.  
> 
> Makes sense.

Nod.

> 
> > Again, I can't really tell what is behind yet. Thus it may
> > be completely irrelevant.
> > 
> > 
> > Regards,
> > Halil
> >   
> > > +};
> > > 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;
> > > +};
> > >   


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