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

[David Hildenbrand]

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).

-- 
Thanks,

David / dhildenb