[virtio-dev] Re: [PATCH] content: Replace guest OS with driver

[Cornelia Huck <cohuck@redhat.com>]

On Tue, May 16 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, May 16, 2023 at 06:01:39AM +0300, Parav Pandit wrote:
>> diff --git a/content.tex b/content.tex
>> index 9df81b8..417d476 100644
>> --- a/content.tex
>> +++ b/content.tex
>> @@ -26,10 +26,10 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>>  following bits are defined (listed below in the order in which
>>  they would be typically set):
>>  \begin{description}
>> -\item[ACKNOWLEDGE (1)] Indicates that the guest OS has found the
>> +\item[ACKNOWLEDGE (1)] Indicates that the driver has found the
>>    device and recognized it as a valid virtio device.
>>  
>> -\item[DRIVER (2)] Indicates that the guest OS knows how to drive the
>> +\item[DRIVER (2)] Indicates that the driver knows how to drive the
>>    device.
>>    \begin{note}
>>      There could be a significant (or infinite) delay before setting
>
> Actually, there is a subtle difference here that this is losing.
> "guest OS" really refers to e.g. Linux virtio core code here.
>
>
> ACKNOWLEDGE and DRIVER are used by virtio core.
>
> ACKNOWLEDGE tells you virtio core attached to device, and DRIVER
> tells you core found a device specific driver.
>
>
>
> If you really want to make things better, let's find a way to explain
> all this.

Agreed, this is a really old part of the spec, and likely had been
written with the Linux device probing sequence in mind.

Basically, we want to distinguish between "something on the driver side
has discovered the device" and "something on the driver side knows how
to drive this specific device". If we consider "driver" as a catch-all
of the whole thing talking to a device, we need to be extra descriptive
(and we can add examples, as this is a non-normative section.)

For ACKNOWLEDGE, maybe "indicates that the driver has discovered the
device and recognized it as a valid virtio device" (i.e. mostly what we
have now), but also add "For example, this can indicate that non-device
specific virtio driver code has attached to the device."

For DRIVER, maybe "indicates that the driver has discovered that it
knows how to drive this device specifically. For example, this can
indicate that device-specific driver code has attached to the device."

Probably need some more overhaul :) Not an editorial change in any case.

>> @@ -473,13 +473,13 @@ \section{Device Initialization}\label{sec:General Initialization And Device Oper
>>  \begin{enumerate}
>>  \item Reset the device.
>>  
>> -\item Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>> +\item Set the ACKNOWLEDGE status bit: the driver has noticed the device.
>>  
>> -\item Set the DRIVER status bit: the guest OS knows how to drive the device.
>> +\item Set the DRIVER status bit: the driver knows how to drive the device.
>
> besides the above, "drivers knows how to drive" sounds bad.
>
>>  \item\label{itm:General Initialization And Device Operation /
>>  Device Initialization / Read feature bits} Read device feature bits, and write the subset of feature bits
>> -   understood by the OS and driver to the device.  During this step the
>> +   understood by the driver to the device.  During this step the
>
> Again the "the OS" here referred to core virtio (e.g. ring features).
> Less of a problem to remove but if we come up with
> a better terminology for ACKNOWLEDGE/DRIVER then I guess we can use it
> here, too.

Hm, I'm not sure how far we need to distinguish between generic and
device-specific features in this case. The "driver" as the whole entity
driving the device needs to decide on the subset; at this stage, it does
not really matter which parts of the driver code accepted which
feature. We probably want to be explicit that features are ring,
transport, and device features, though.


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