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

[Halil Pasic <pasic@linux.ibm.com>]

On Wed, 10 Jun 2020 12:24:14 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 10.06.20 12:07, David Gibson wrote:
> > 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.
> > 
> 
> Reading along I got the impression that we want to glue the availability
> of CPU features to other QEMU cmdline parameters (besides the
> accelerator). ("to set (default) cpu properties as well"). If we are
> talking about other CPU properties not expressed as CPU features (e.g.,
> -cpu X,Y=on ...), then there is no issue.
> 

I share the concerns broght forward by David.