Re: [PATCH v8 1/9] virtio: document forward compatibility guarantees

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Nov 22 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Nov 21, 2022 at 04:24:26PM +0100, Cornelia Huck wrote:
>> On Sun, Nov 20 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >  \drivernormative{\subsection}{Feature Bits}{Basic Facilities of a Virtio Device / Feature Bits}
>> >  The driver MUST NOT accept a feature which the device did not offer,
>> >  and MUST NOT accept a feature which requires another feature which was
>> >  not accepted.
>> >  
>> > +The driver MUST validate the feature bits offered by the device.
>> > +The driver MUST ignore and MUST NOT accept feature bits not
>> > +described in this specification, reserved feature bits and
>> > +feature bits reserved or not supported for the specific transport
>> > +or the specific device type.
>> 
>> I think we can mandate that the driver MUST NOT accept these feature
>> bits; but can we mandate that it MUST ignore them? Alternatively, it
>> could also set the FAILED status bit.
>
> Well if drivers assume there are no
> new features then we can't add new ones and forward compatibility
> breaks. We always assumed drivers ignore features they don't
> understand.  This is why I propose mandating this strongly and
> I feel it's okay to add such a requirement even at this late stage
> anything violating this would have been broken many times already.
> No?

Hm, right. Let's keep this as MUST.

>
>> Also, what does "specific device type" mean in this case? Does it refer
>> to a driver that supports devices virtio-foo, virtio-bar, and
>> virtio-baz, and so needs to ignore a feature that is only valid for
>> virtio-foo, but not for virtio-bar or virtio-baz, if it is offered for
>> anything that is not virtio-foo?
>
> For example, virtio-rng has no features, driver must ignore
> all device specific feature bits and only negotiate generic
> and transport specific feature bits.

Isn't that the same as a feature bit that the driver does not know?
Feature bit 0 means different things for different device types, and for
virtio-rng, it's simply an undefined feature (as of now).

> How would you put it better?

Let me try:

The driver MUST ignore and MUST NOT accept any feature bit that is
\begin{itemize}
\item not described in this specification,
\item marked as reserved,
\item not supported for the specific transport,
\item not defined for the device type.
\end{itemize}