[virtio-comment] Re: [PATCH v7] virtio-net: Add support for the flexible driver notification structure.

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 18 Mar 2020 18:29:19 +0000
Vitaly Mireyno <vmireyno@marvell.com> wrote:

> Currently, the driver notification (available buffer notification) has a fixed structure, which includes the virtqueue number.
> For PCI transport, if notify_off_multiplier > 0, the VQ number can potentially be derived by the device from the Queue Notify address, so providing virtqueue number as part of the data may be redundant.
> Some devices benefit from receiving an additional data with driver notifications. This data can optionally replace the vqn field in the driver notification structure.
> In its simplest form, it would be sufficient for this data to be a per-device constant value.
> 
> Changes from v6:
>  * The feature definition is moved to a generic section, referring to PCI-specific details.

Thanks, this looks much better.

> 
> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
> ---
>  content.tex | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index b91a132..163c47e 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -965,6 +965,8 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options
>  struct virtio_pci_notify_cap {
>          struct virtio_pci_cap cap;
>          le32 notify_off_multiplier; /* Multiplier for queue_notify_off. */
> +        le16 notify_data; /* Data to be placed in the vqn field */
> +        le16 padding; /* Pad to a dword */
>  };
>  \end{lstlisting}
>  
> @@ -984,6 +986,16 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options
>  the same Queue Notify address for all queues.
>  \end{note}
>  
> +\field{notify_data} is the data that the driver will set in the available
> +buffer notification (instead of the virtqueue number), if
> +VIRTIO_NET_F_NOTIF_CONFIG_DATA has been negotiated.
> +
> +\begin{note}
> +If \field{notify_off_multiplier} > 0, the virtqueue number can potentially be
> +derived by the device from the Queue Notify address, so \field{vqn} may be
> +redundant.
> +\end{note}
> +
>  \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.
>  
> @@ -1020,6 +1032,9 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options
>  cap.length >= queue_notify_off * notify_off_multiplier + 4
>  \end{lstlisting}
>  
> +If the device offers VIRTIO_NET_F_NOTIF_CONFIG_DATA, it MUST set
> +\field{notify_off_multiplier} > 0 in at least one capability.

Maybe s/capability/notification capability/ ?

> +
>  \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
> @@ -1519,6 +1534,12 @@ \subsubsection{Available Buffer Notifications}\label{sec:Virtio Transport Option
>  See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
>  for how to calculate the Queue Notify address.
>  
> +\drivernormative{\paragraph}{Available Buffer Notifications}{Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}
> +If VIRTIO_NET_F_NOTIF_CONFIG_DATA has been negotiated and
> +\field{notify_off_multiplier} > 0, the driver MUST set the 'virtqueue number'
> +field of the available buffer notification structure to the \field{notify_data}
> +value.
> +
>  \subsubsection{Used Buffer Notifications}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Used Buffer Notifications}
>  
>  If a used buffer notification is necessary for a virtqueue, the device would typically act as follows:
> @@ -2895,6 +2916,10 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
>      channel.
>  
> +\item[VIRTIO_NET_F_NOTIF_CONFIG_DATA(57)] Driver provides extra data with
> +    available buffer notifications, to aid in notification processing by the
> +    device.
> +
>  \item[VIRTIO_NET_F_GUEST_HDRLEN(59)] Driver can provide the exact \field{hdr_len}
>      value. Device benefits from knowing the exact header length.
>  
> @@ -4180,6 +4205,33 @@ \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
>  See \ref{sec:Basic
>  Facilities of a Virtio Device / Virtqueues / Message Framing}.
>  
> +\subsubsection{Driver Notifications}\label{sec:Device Types / Network Device / Device Operation / Driver Notifications}
> +As mentioned in section \ref{sec:Virtqueues / Driver notifications}, when the
> +driver is required to send an available buffer notification to the device, it
> +always sends the virtqueue number to be notified. The method of delivering
> +notifications is transport specific. With the PCI transport, the device can
> +optionally provide a distinct per-virtqueue address for driver notifications
> +(Queue Notify address). In this case, the virtqueue number can potentially be
> +derived by the device from the Queue Notify address. For devices, that provide
> +per-virtqueue Queue Notify addresses and are able to derive the virtqueue number
> +from that address, the virtqueue number in the notification data is redundant.

I would keep the remark that the queue number might be redundant with
the PCI transport.

> +On the other hand, some devices can benefit from receiving additional data with
> +driver notifications. An example could be a hardware device implementing multiple
> +protocols (with virtio being one of them), so the extra notification data could
> +serve as a notification type indication or a protocol indication.

Hm... what about replacing the whole text above with

"Driver notifications (\ref{sec:Virtqueues / Driver notifications})
always contain at least the virtqueue number for which buffers are
available. Some devices can benefit from receiving additional data with
driver notifications. For example, a hardware device implementing
multiple protocols (with virtio being one of them) could benefit from a
notification type indication or a protocol indication."

> +
> +VIRTIO_NET_F_NOTIF_CONFIG_DATA feature can be used with transports that support
> +a similar driver notifications delivery method, and it allows providing extra
> +data by the driver to the device. The definition of the data, that the driver
> +provides to the device instead of the virtqueue number, is transport specific.

What about:

"The VIRTIO_NET_F_NOTIF_CONFIG_DATA feature indicates the availability
of such additional data. The definition of the data to be provided by
the driver during driver notification and the method for delivering it
is transport specific."

> +
> +For more details about driver notifications over PCI see \ref{sec:Virtio
> +Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}.
> +
> +\drivernormative{\paragraph}{Driver Notifications}{Device Types / Network Device / Device Operation / Driver Notifications}
> +The driver SHOULD accept the VIRTIO_NET_F_NOTIF_CONFIG_DATA feature if it has
> +been offered.

I'd also add

"If the VIRTIO_NET_F_NOTIF_CONFIG_DATA feature has been negotiated, the
driver MUST provide the transport-specific additional data with driver
notifications."

> +
>  \section{Block Device}\label{sec:Device Types / Block Device}
>  
>  The virtio block device is a simple virtual block device (ie.
> --
> 


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