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

[Michael S. Tsirkin]

On Tue, Jun 09, 2020 at 05:47:47PM +0200, Claudio Imbrenda 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.

So my suggestion was basically a flag that sets both the
CPU model feature and the virtio feature.



> What __could__ be done is to prevent qemu from even starting if an
> incompatible device is specified together with PV.
>
> Another option is to disable PV at the qemu level if an incompatible
> device is present. This will have the effect that trying to boot a
> secure guest will fail mysteriously, which is IMHO also not too great.
> 
> do we really have that many incompatible devices?
>