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

[Pierre Morel]

On 2020-06-09 11:41, Halil Pasic wrote:
> On Tue, 9 Jun 2020 08:44:02 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
>
> > On Mon, 8 Jun 2020 19:00:45 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:

...snip...

> > I mean, can a virtio devices without IOMMU_PLATFORM act as a transition
> > blocker (i.e. an attempt by a guest to move to pv would fail?)
>
> 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.


The guest image is encrypted and the transition to PV is done by the 
stage 3 boot loader, part of the loaded image, before Linux is started.
At this moment the guest is not aware of the virtio devices.

Regards,
Pierre

--
Pierre Morel
IBM Lab Boeblingen