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

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Date:   Mon, 4 Oct 2021 16:01:12 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cornelia Huck <cohuck@...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: [virtio-dev] Re: [RFC PATCH 1/1] virtio: write back features
 before verify

On Mon, Oct 04, 2021 at 05:45:06PM +0200, Cornelia Huck wrote:
> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Mon, Oct 04, 2021 at 04:27:23PM +0200, Cornelia Huck wrote:
> >> 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?
> >
> > 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?

Maybe "as long as". However Halil implied that some features are not
turned off properly if that happens. Halil could you pls provide
some examples?

-- 
MST