From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 4 Oct 2021 16:01:12 -0400 From: "Michael S. Tsirkin" Subject: Re: [virtio-dev] Re: [RFC PATCH 1/1] virtio: write back features before verify Message-ID: <20211004160005-mutt-send-email-mst@kernel.org> References: <87fstm47no.fsf@redhat.com> <20211002141351-mutt-send-email-mst@kernel.org> <20211003070030.658fc94e.pasic@linux.ibm.com> <20211003021027-mutt-send-email-mst@kernel.org> <20211003032253-mutt-send-email-mst@kernel.org> <87ee912e45.fsf@redhat.com> <20211004083455-mutt-send-email-mst@kernel.org> <878rz83lx0.fsf@redhat.com> <20211004110152-mutt-send-email-mst@kernel.org> <87zgro23r1.fsf@redhat.com> MIME-Version: 1.0 In-Reply-To: <87zgro23r1.fsf@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline To: Cornelia Huck Cc: Halil Pasic , Jason Wang , Xie Yongji , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, markver@us.ibm.com, Christian Borntraeger , linux-s390@vger.kernel.org, virtio-dev@lists.oasis-open.org List-ID: On Mon, Oct 04, 2021 at 05:45:06PM +0200, Cornelia Huck wrote: > On Mon, Oct 04 2021, "Michael S. Tsirkin" wrote: > > > On Mon, Oct 04, 2021 at 04:27:23PM +0200, Cornelia Huck wrote: > >> On Mon, Oct 04 2021, "Michael S. Tsirkin" wrote: > >> > >> > On Mon, Oct 04, 2021 at 02:01:14PM +0200, Cornelia Huck wrote: > >> >> On Sun, Oct 03 2021, "Michael S. Tsirkin" 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