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

[Christian Borntraeger]

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"?