Re: [virtio-dev] Re: [RFC PATCH 1/1] virtio: write back features before verify

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Oct 04, 2021 at 04:27:23PM +0200, Cornelia Huck wrote:
>> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> 
>> > On Mon, Oct 04, 2021 at 02:01:14PM +0200, Cornelia Huck wrote:
>> >> On Sun, Oct 03 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >> > @@ -160,6 +163,33 @@ \subsection{Legacy Interface: A Note on Feature
>> >> >  Specification text within these sections generally does not apply
>> >> >  to non-transitional devices.
>> >> >  
>> >> > +\begin{note}
>> >> > +The device offers different features when used through
>> >> > +the legacy interface and when operated in accordance with this
>> >> > +specification.
>> >> > +\end{note}
>> >> > +
>> >> > +Transitional drivers MUST use Devices only through the legacy interface
>> >> 
>> >> s/Devices only through the legacy interface/devices through the legacy
>> >> interface only/
>> >> 
>> >> ?
>> >
>> > Both versions are actually confused, since how do you
>> > find out that device does not offer VIRTIO_F_VERSION_1?
>> >
>> > I think what this should really say is
>> >
>> > Transitional drivers MUST NOT accept VIRTIO_F_VERSION_1 through
>> > the legacy interface.
>> 
>> Ok, that makes sense.
>> 
>> Would it make sense that transitional drivers MUST accept VERSION_1
>> through the non-legacy interface? Or is that redundant?
>
> We already have:
>
> A driver MUST accept VIRTIO_F_VERSION_1 if it is offered.

Yep, so it is redundant.

>
>
>> >
>> >
>> > Does linux actually satisfy this? Will it accept VIRTIO_F_VERSION_1
>> > through the legacy interface if offered?
>> 
>> I think that the Linux drivers will not operate on feature bit 32+ if
>> they are in legacy mode?
>
>
> Well ... with PCI there's no *way* for host to set bit 32 through
> legacy. But it might be possible with MMIO/CCW. Can you tell me
> what happens then?

ccw does not support accessing bit 32+, either. Not sure about mmio.

>
>
>> >> 
>> >> Generally, looks good to me.
>> >
>> > Do we want to also add explanation that features can be
>> > changed until FEATURES_OK?
>> 
>> I always considered that to be implict, as feature negotiation is not
>> over until we have FEATURES_OK. Not sure whether we need an extra note.
>
> Well Halil here says once you set a feature bit you can't clear it.
> So maybe not ...

Ok, so what about something like

"If FEATURES_OK is not set, the driver MAY change the set of features it
accepts."

in the device initialization section?