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

[David Gibson]

On Wed, Jun 10, 2020 at 09:22:45AM +0200, David Hildenbrand wrote:
> On 10.06.20 06:31, David Gibson wrote:
> > On Tue, Jun 09, 2020 at 12:44:39PM -0400, Michael S. Tsirkin wrote:
> >> 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).
> >>
> >> So if we want to keep it user friendly, there could be
> >> protection property with values on/off/auto, and auto
> >> would poke at host capability to figure out whether
> >> it's supported.
> >>
> >> Both virtio and CPU would inherit from that.
> > 
> > Right, that's what I have in mind for my 'host-trust-limitation'
> > property (a generalized version of the existing 'memory-encryption'
> > machine option).  My draft patches already set virtio properties
> > accordingly, it should be possible to set (default) cpu properties as
> > well.
> 
> No crazy CPU model hacks please (at least speaking for the s390x).

Uh... I'm not really sure what you have in mind here.

-- 
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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