Cornelia Huck
2021-Oct-04 15:45 UTC
[virtio-dev] Re: [RFC PATCH 1/1] virtio: write back features before verify
On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst at 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 at 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 at 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?
Michael S. Tsirkin
2021-Oct-04 20:01 UTC
[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 at 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 at 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 at 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?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