Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV

[David Gibson]

On Tue, Jun 09, 2020 at 06:28:39PM +0200, Halil Pasic wrote:
> On Tue, 9 Jun 2020 17:47:47 +0200
> Claudio Imbrenda <imbrenda@linux.ibm.com> wrote:
> 
> > On Tue, 9 Jun 2020 11:41:30 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> > 
> > [...]
> > 
> > > I don't know. Janosch could answer that, but he is on vacation. Adding
> > > Claudio maybe he can answer. My understanding is, that while it might
> > > be possible, it is ugly at best. The ability to do a transition is
> > > indicated by a CPU model feature. Indicating the feature to the guest
> > > and then failing the transition sounds wrong to me.
> > 
> > I agree. If the feature is advertised, then it has to work. I don't
> > think we even have an architected way to fail the transition for that
> > reason.
> > 
> > What __could__ be done is to prevent qemu from even starting if an
> > incompatible device is specified together with PV.
> 
> AFAIU, the "specified together with PV" is the problem here. Currently
> we don't "specify PV" but PV is just a capability that is managed by the
> CPU model (like so many other). I.e. the fact that the
> visualization environment is capable providing PV (unpack facility
> available), and the fact, that the end user didn't fence the unpack
> facility, does not mean, the user is dead set to use PV.
> 
> My understanding is, that we want PV to just work, without having to
> put together a peculiar VM definition that says: this is going to be
> used as a PV VM.

Having had a similar discussion for POWER, I no longer think this is a
wise model.  I think we want to have an explicit "allow PV" option -
but we do want it to be a *single* option, rather than having to
change configuration of a whole bunch of places.

My intention is for my 'host-trust-limitation' series to accomplish
that.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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