Re: [PATCH V4] virtio-transport: Clarify requirements

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Jun 04 2024, Viresh Kumar <viresh.kumar@linaro.org> wrote:

> The virtio documentation currently doesn't define any generic
> requirements that are applicable to all transports. They can be useful
> while adding support for a new transport.
>
> This commit tries to define the same.
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> V3->V4:
> - Remove the normative sections and use direct speech.
> - Change wording at few places.
>
> V2->V3:
> - Minor fixes.
> - Added Reviewed by from Alex.
>
> V1->V2:
> - Lot of changes after discussions with Alex and Cornelia.
> - Almost a rewrite of the first commit.
> - Add Transport normative sections.
>
>  content.tex | 80 +++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 78 insertions(+), 2 deletions(-)
>
> diff --git a/content.tex b/content.tex
> index 0a62dce5f65f..23eea890d9b7 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -631,8 +631,84 @@ \section{Device Cleanup}\label{sec:General Initialization And Device Operation /
>  
>  \chapter{Virtio Transport Options}\label{sec:Virtio Transport Options}
>  
> -Virtio can use various different buses, thus the standard is split
> -into virtio general and bus-specific sections.
> +Devices and drivers can use different transport methods to enable
> +interaction, for example PCI, MMIO, or Channel I/O. The transport
> +methods define various aspects of the communication between the device
> +and the driver, like device discovery, exchanging capabilities,
> +interrupt handling, data transfer, etc. For example, in a host/guest
> +architecture, the host might expose a device to the guest on a PCI bus,
> +and the guest will use a PCI-specific driver to interact with it.
> +
> +The standard contains sections describing the transport-agnostic parts
> +of virtio, and sections describing how individual transports implement
> +virtio.
> +
> +\section{Virtio Transport Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements}
> +

It might be useful to put some kind of lead-in here, maybe something
like

"There are some mechanisms that any transport is required to implement,
and some requirements that devices and drivers are required to follow."

> +\subsection{Transport Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements / Transport Requirements}
> +
> +The transport provides a mechanism for the driver to discover the
> +device.

Maybe "A transport", or "Any transport"? I think "The transport" is a
bit awkward if we're not talking about a specific transport, but
something any transport is supposed to do.

> +
> +The transport provides a mechanism for the driver to identify the device
> +type.
> +
> +The transport provides a mechanism for communicating virtqueue
> +configurations between the device and the driver.
> +
> +The transport allows multiple virtqueues per device. The number of
> +virtqueues for a pair of device-driver are governed by the individual
> +device protocol.
> +
> +The transport provides a mechanism that the device and the driver use to
> +access memory for implementing virtqueues.
> +
> +The transport provides a mechanism for the device to notify the driver
> +and a mechanism for the driver to notify the device, for example
> +regarding availability of a buffer on the virtqueue.
> +
> +The transport provides a mechanism for the driver to initiate a reset
> +of the virtqueues and device.
> +
> +The transport provides a mechanism for the driver to read the device
> +status. The transport MUST provide a mechanism for the driver to change

s/MUST provide/provides/

> +the device status.
> +
> +The transport provides a mechanism to implement configuration space
> +between the device and the driver.
> +
> +\subsection{Device Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements / Device Requirements}
> +
> +The device keeps any data associated with a device-initiated transaction
> +accessible to the driver until the driver acknowledges the transaction
> +to be complete.
> +
> +The device doesn't access the contents of a virtqueue before the driver
> +notifies, in a transport defined way, the device that the virtqueue is
> +ready to be accessed.
> +
> +The device doesn't access or modify buffers on a virtqueue after it has
> +notified the driver about their availability.
> +
> +The device resets itself and the virtqueues if requested by the driver, in a
> +transport defined way, if the transport provides such a method.
> +
> +\subsection{Driver Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements / Driver Requirements}
> +
> +The driver acknowledges device notifications, as mandated by the
> +transport.
> +
> +The driver doesn't access virtqueue contents before the device notifies
> +about the readiness of the same.
> +
> +The driver accesses queued buffers after the device has processed them
> +and notified the driver of their availability. This mechanism is
> +transport defined.
> +
> +The driver asks the device to reset itself and the virtqueues if, for
> +example, the driver times out waiting for a notification from the device
> +for a previously queued request.
> +
>  
>  \input{transport-pci.tex}
>  \input{transport-mmio.tex}