[libvirt] s390: change default cpu model to host-model?

[Christian Borntraeger]

On 08.11.19 12:52, Daniel P. Berrangé wrote:
> On Fri, Nov 08, 2019 at 12:49:23PM +0100, Christian Borntraeger wrote:
>>
>>
>> On 08.11.19 12:43, Daniel P. Berrangé wrote:
>>> On Mon, Nov 04, 2019 at 11:49:01AM +0100, David Hildenbrand wrote:
>>>> On 02.11.19 11:32, Daniel P. Berrangé wrote:
>>>>> On Fri, Nov 01, 2019 at 06:43:16PM +0100, Christian Borntraeger wrote:
>>>>>> On the KVM forum I have discussed the default cpu model mode on s390.
>>>>>> Right now if the xml does not specify anything, libvirt defaults to
>>>>>> not specifying anything on the qemu command line (no -cpu statement)
>>>>>> which is the equivalent of -cpu host for s390 which is equivalent to
>>>>>> host-passthrough. While this enables all features it does not provide
>>>>>> any migration safety by default.
>>>>>>
>>>>>> So in fact we are kind of "broken" right now when it comes to safery.
>>>>>>
>>>>>> So we discussed that it would make sense that an empty xml should actually
>>>>>> be defaulted to host-model, which results in - as of today - the same guest
>>>>>> features but in a migration safe way.
>>>>>>
>>>>>> There is another change planned right now to actually make the cpu model
>>>>>> present in an xml if none was specified. So we could actually do this change
>>>>>> before, together  or after te other. Jiri and I think it probably makes most
>>>>>> sense to have both changes at the same time (in terms of libvirt version).
>>>>>>
>>>>>> Does anyone see an issue with changing the default model mode to "host-model"
>>>>>> if the xml does not specify anything else?
>>>>>
>>>>> Changing from "host-passthrough" to "host-model" is not a huge difference,
>>>>> but it is none the less a guest ABI change. "host-passthrough" doesn't
>>>>> provide migration safety in the face of differing hardware, it should still
>>>>> be valid for people with homogeneous hardware. So changing the model will
>>>>> potentially break some existing usage.
>>>>
>>>> I guess on s390x this is not the case ("-cpu host", no "-cpu", and passing
>>>> the expanded "host" model will result in the same guest ABI, in contrast to
>>>> x86 AFAIK). There is this special case, though, where we have old QEMUs
>>>> without CPU model support. Not sure how to deal with that, then.
>>>
>>> I'm still not sure I understand the s390 CPU ABI rules.
>>>
>>> Current libvirt, no <cpu>, and thus no -cpu.
>>>
>>> IIUC this is functionally identical to using "-cpu host" and/or
>>> <cpu mode="host-passthrough"/>
>>>
>>> If you are using "-cpu host" / <cpu mode="host-passthrough"> can you
>>> live migrate to another host with identical physical CPUs + firmware ?
>>>
>>>
>>> Assuming this is possible, then, can you live migrate a QEMU guest
>>> booted with <cpu mode="host-passthrough">, to a QEMU guest booted
>>> with <cpu mode="host-model">  ?
>>
>> Not sure I understand your question. With "can", do  you mean "the guest
>> has the same guest visible CPU features and types"?
> 
> Yes, I mean the migration should succeed from QEMU's POV and additionally
> the guest OS should not see any change in CPU ABI exposed from the host.

The guest ABI is the same and migration also seems to work. 
I think your concern boils down to the case that source and target
have a different libvirt version (but qemu and kernel and firmware
and hardware are identical). So for that case this change would break
things if host-model and host-passthrough are not identical. 

So as of today we have no concern. 

Now: Would it be a concern if a future QEMU decides to change that
equivalence somehow?