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

[David Hildenbrand]

On 10.06.20 15:19, Viktor Mihajlovski wrote:
> 
> 
> On 6/10/20 12:24 PM, David Hildenbrand 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.
>>
> 
> The reason that the capability to run in PV mode is expressed in the CPU
> model is that this capability *is* provided by the CPU in terms of
> available instructions. I wouldn't see a benefit in providing
> a meta-property that needs to be synced with the CPU model.
> 
> So, if something has to be concluded from the fact that a VM
> could run in PV mode, that decision should be derived from the
> CPU model.
> 

Exactly.

-- 
Thanks,

David / dhildenb