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

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Message-ID: <878rz83lx0.fsf@redhat.com>
Date:   Mon, 04 Oct 2021 16:27:23 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, markver@...ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, virtio-dev@...ts.oasis-open.org
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

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

> On Mon, Oct 04, 2021 at 02:01:14PM +0200, Cornelia Huck wrote:
>> On Sun, Oct 03 2021, "Michael S. Tsirkin" <mst@...hat.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?

>
>
> 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?

>> 
>> 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.