On Wed, Nov 23, 2022 at 10:21:52AM +0100, Cornelia Huck wrote:
> On Tue, Nov 22 2022, "Michael S. Tsirkin" <> wrote: > >
> On Mon, Nov 21, 2022 at 04:24:26PM +0100, Cornelia Huck wrote: > >> On Sun, Nov 20 2022, "Michael S. Tsirkin" <> 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 > egin{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} ok, thanks!
