RE: [PATCH v14 03/11] content: Rename confusing queue_notify_data and vqn names

[Parav Pandit <parav@nvidia.com>]

> From: Halil Pasic <pasic@linux.ibm.com>
> Sent: Friday, April 21, 2023 6:38 PM
> >
> >  \begin{description}
> > -\item [vqn] VQ number to be notified.
> > +\item [vq_index_config_data] Either virtqueue index or device
> > +supplied
> 
> Apparently this our name for either index or notification config data, but I don't
> think we ever use it.
> 
Right because this is generic description without the field data type and their endianness and transport specific information.
So, it uses an abstract term.

> I guess, there is a two to think about it:
> * Notifications are mostly transport specific. In particular what
>   data structures are used for the virtqueue notifications and how
>   they are passed, that is entirely transport specific. So the 'layout'
>   and the normative statements describing the details should be in the
>   transport sections.
Right. This addition did not add any normative. They are in transport section as you say.

> * The facility bits and the capabilities are basically common. I.e. if
>   MMIO or/and Channel I/O were to ever support VIRTIO_F_NOTIF_CONFIG
> data
>   (by providing means for transporting the virtqueue notification config
>   data from the device to the driver), what you call the notification
>   value would be either virtqueue index or this notification config data.
> 
> You are taking the first approach here, and I'm fine with that. IMHO before we
> had the second approach implemented with vqn. I'm fine with taking the first
> approach as well, but your commit message says "rename ambiguous vqn to a
> union of vq_index and vq_config_data" which for me implies you actually
> intended to take the second approach.
> 
Those are renamed where the actual vqn is defined in the structure like below.
In basic description section, it is just describing the functionality.

> > @@ -1,5 +1,5 @@
> >  le32 {
> > -	vqn : 16;
> > +	vq_index: 16; /* previously known as vqn */
> >  	next_off : 15;
> >  	next_wrap : 1;
> >  };
> > diff --git a/notifications-pci-le.c b/notifications-pci-le.c new file
> > mode 100644 index 0000000..ef60d15
> > --- /dev/null
> > +++ b/notifications-pci-le.c
> > @@ -0,0 +1,8 @@
> > +le32 {
> > +	union {
> > +		vq_index: 16; /* Used if VIRTIO_F_NOTIF_CONFIG_DATA not
> negotiated */
> > +		vq_notif_config_data: 16; /* Used if
> VIRTIO_F_NOTIF_CONFIG_DATA negotiated */
> > +	};
> > +	next_off : 15;
> > +	next_wrap : 1;
> > +};
> > diff --git a/transport-pci.tex b/transport-pci.tex index
> > 5d98467..04a9a80 100644
> > --- a/transport-pci.tex
> > +++ b/transport-pci.tex
> > @@ -319,7 +319,7 @@ \subsubsection{Common configuration structure
> layout}\label{sec:Virtio Transport
> >          le64 queue_desc;                /* read-write */
> >          le64 queue_driver;              /* read-write */
> >          le64 queue_device;              /* read-write */
> > -        le16 queue_notify_data;         /* read-only for driver */
> > +        le16 queue_notif_config_data;   /* read-only for driver */
> >          le16 queue_reset;               /* read-write */
> >  };
> >  \end{lstlisting}
> > @@ -388,17 +388,21 @@ \subsubsection{Common configuration structure
> > layout}\label{sec:Virtio Transport  \item[\field{queue_device}]
> >          The driver writes the physical address of Device Area here.  See section
> \ref{sec:Basic Facilities of a Virtio Device / Virtqueues}.
> >
> > -\item[\field{queue_notify_data}]
> > +\item[\field{queue_notif_config_data}]
> >          This field exists only if VIRTIO_F_NOTIF_CONFIG_DATA has been
> negotiated.
> > -        The driver will use this value to put it in the 'virtqueue number' field
> > -        in the available buffer notification structure.
> > +        The driver will use this value when driver sends available buffer
> > +        notification to the device.
> >          See section \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-
> specific Initialization And Device Operation / Available Buffer Notifications}.
> >          \begin{note}
> >          This field provides the device with flexibility to determine how
> virtqueues
> >          will be referred to in available buffer notifications.
> > -        In a trivial case the device can set \field{queue_notify_data}=vqn. Some
> devices
> > -        may benefit from providing another value, for example an internal
> virtqueue
> > -        identifier, or an internal offset related to the virtqueue number.
> > +        In a trivial case the device can set \field{queue_notif_config_data} to
> > +        virtqueue index. Some devices may benefit from providing another
> value,
> > +        for example an internal virtqueue identifier, or an internal offset
> > +        related to the virtqueue index.
> > +        \end{note}
> > +        \begin{note}
> > +        This field is previously known as queue_notify_data.
> >          \end{note}
> >
> >  \item[\field{queue_reset}]
> > @@ -468,7 +472,9 @@ \subsubsection{Common configuration structure
> > layout}\label{sec:Virtio Transport
> >
> >  \drivernormative{\paragraph}{Common configuration structure
> > layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device
> > Layout / Common configuration structure layout}
> >
> > -The driver MUST NOT write to \field{device_feature}, \field{num_queues},
> \field{config_generation}, \field{queue_notify_off} or
> \field{queue_notify_data}.
> > +The driver MUST NOT write to \field{device_feature},
> > +\field{num_queues}, \field{config_generation},
> > +\field{queue_notify_off} or \field{queue_notif_config_data}.
> >
> >  If VIRTIO_F_RING_PACKED has been negotiated,  the driver MUST NOT
> > write the value 0 to \field{queue_size}.
> > @@ -1035,13 +1041,22 @@ \subsubsection{Available Buffer
> > Notifications}\label{sec:Virtio Transport Option
> >
> >  When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,  the driver
> > sends an available buffer notification to the device by writing -the
> > 16-bit virtqueue index -of this virtqueue to the Queue Notify address.
> > +only the 16-bit notification value to the Queue Notify address of the
> 
> 
> You introduce the term notification value. Which basically means virtqueue
> index or virtqueue notification config data. 
Right and it is captured in the union fields.

> Which is basically what is defined
> where you define vq_index_config_data (the latter is never ever used again).
> 
vq_index_config_data is abstract term without any bit fields, width, endinenss and without any attachment to transport details.

> > +virtqueue. A notification value depends on the negotiation of
> > +VIRTIO_F_NOTIF_CONFIG_DATA.
> >
> > -When VIRTIO_F_NOTIFICATION_DATA has been negotiated, -the driver
> > sends an available buffer notification to the device by writing -the
> > following 32-bit value to the Queue Notify address:
> > -\lstinputlisting{notifications-le.c}
> > +\item If VIRTIO_F_NOTIFICATION_DATA has been negotiated, the driver
> > +sends an
> 
> The leading \item is probably not intended. Breaks the build and makes no
> sense.
Yes. it was not intended. Good catch.
Fixing in v15.