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

[Claudio Imbrenda]

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.

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?