Re: [PATCH] virtio-transport: Clarify requirements

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jan 29 2024, Viresh Kumar <viresh.kumar@linaro.org> wrote:

> On 18-12-23, 15:02, Cornelia Huck wrote:
>> Other things that probably should be mandatory:
>> 
>> - A way for the driver to change the device status. Reading it would
>>   probably be a strong SHOULD.
>> - A way to implement config space. (For example, channel devices don't
>>   have a config space or anything similar enough to repurpose, so I had
>>   to invent a mechanism for ccw... maybe future transports will be in
>>   the same boat.)
>
> How about these changes now, before I post them again:
>
> diff --git a/content.tex b/content.tex
> index 0a62dce5f65f..2a86b1041747 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -631,8 +631,81 @@ \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 is split into sections describing general virtio
> +implementation and transport-specific sections.
> +
> +\section{Virtio Transport Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements}
> +
> +The transport MUST provide a mechanism for the driver to discover the
> +device.
> +
> +The transport must provide a mechanism for the driver to identify the

s/must/MUST/

> +device type.
> +
> +The transport MUST provide a mechanism for communicating virtqueue
> +configurations between the device and the driver.
> +
> +The transport MUST allow multiple virtqueues per device. The number of
> +virtqueues for a pair of device-driver are governed by the individual
> +device protocol.
> +
> +The transport MUST provide a mechanism that the device and the driver
> +use to access memory for implementing virtqueues.
> +
> +The transport MUST provide 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 MAY provide a mechanism for the driver to initiate a
> +reset of the virtqueues and device.
> +
> +The transport MUST provide a mechanism for the driver to read the
> +device status. The transport SHOULD provide a mechanism for the driver to
> +change the device status.

I think the other way around? CCW only gained support for reading the
status with revision 2.

> +
> +The transport MUST provide a mechanism to implement config space between
> +the device and the driver.
> +
> +\devicenormative{\subsection}{Virtio Transport Requirements}{Virtio Transport Options}
> +
> +Any data associated with a device-initiated transaction MUST remain
> +accessible to the driver until the driver acknowledges the transaction
> +to be complete.

Maybe better "The device MUST keep any data (...) accessible to the
driver..." ?

> +
> +The device MUST NOT 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 MUST NOT access or modify buffers on a virtqueue after it has
> +notified the driver about their availability.
> +
> +The device MUST reset the virtqueues if requested, in a transport
> +defined way, by the driver.

"in a transport defined way, if the transport provides such a method" ?

> +
> +\drivernormative{\subsection}{Virtio Transport Requirements}{Virtio Transport Options}
> +
> +The driver MUST acknowledge device notifications, as mandated by the
> +transport.
> +
> +The driver MUST NOT access virtqueue contents before the device notifies
> +about the readiness of the same.
> +
> +The driver MUST NOT access buffers, after it has added them to the
> +virtqueue and notified the device about their availability. The driver
> +MAY access them after the device has processed them and notified the
> +driver of their availability, in a transport defined way.
> +
> +The driver MAY ask the device to reset 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}
>

These statements need to be hooked up in comformance.tex -- we also need
a way to formulate "transport normative" statements, I think?