[PATCH RFC 0/1] s390x CPU Model Feature Deprecation

[Boris Fiuczynski]

On 3/15/22 4:58 PM, David Hildenbrand wrote:
> On 11.03.22 13:44, Christian Borntraeger wrote:
>>
>>
>> Am 11.03.22 um 10:30 schrieb David Hildenbrand:
>>> On 11.03.22 05:17, Collin Walling wrote:
>>>> The s390x architecture has a growing list of features that will no longer
>>>> be supported on future hardware releases. This introduces an issue with
>>>> migration such that guests, running on models with these features enabled,
>>>> will be rejected outright by machines that do not support these features.
>>>>
>>>> A current example is the CSSKE feature that has been deprecated for some time.
>>>> It has been publicly announced that gen15 will be the last release to
>>>> support this feature, however we have postponed this to gen16a. A possible
>>>> solution to remedy this would be to create a new QEMU QMP Response that allows
>>>> users to query for deprecated/unsupported features.
>>>>
>>>> This presents two parts of the puzzle: how to report deprecated features to
>>>> a user (libvirt) and how should libvirt handle this information.
>>>>
>>>> First, let's discuss the latter. The patch presented alongside this cover letter
>>>> attempts to solve the migration issue by hard-coding the CSSKE feature to be
>>>> disabled for all s390x CPU models. This is done by simply appending the CSSKE
>>>> feature with the disabled policy to the host-model.
>>>>
>>>> libvirt pseudo:
>>>>
>>>> if arch is s390x
>>>>       set CSSKE to disabled for host-model
>>>
>>> That violates host-model semantics and possibly the user intend. There
>>> would have to be some toggle to manually specify this, for example, a
>>> new model type or a some magical flag.
>>
>> What we actually want to do is to disable csske completely from QEMU and
>> thus from the host-model. Then it would not violate the spec.
>> But this has all kind of issues (you cannot migrate from older versions
>> of software and machines) although the hardware still can provide the feature.
>>
>> The hardware guys promised me to deprecate things two generations earlier
>> and we usually deprecate things that are not used or where software has a
>> runtime switch.
>>
>>   From what I hear from you is that you do not want to modify the host-model
>> semantics to something more useful but rather define a new thing (e.g. "host-sane") ?
> 
> My take would be, to keep the host model consistent, meaning, the
> semantics in QEMU exactly match the semantics in Libvirt. It defines the
> maximum CPU model that's runnable under KVM. If a feature is not
> included (e.g., csske) that feature cannot be enabled in any way.
> 
> The "host model" has the semantics of resembling the actual host CPU.
> This is only partially true, because we support some features the host
> might not support (e.g., zPCI IIRC) and obviously don't support all host
> features in QEMU.
> 
> So instead of playing games on the libvirt side with the host model, I
> see the following alternatives:
> 
> 1. Remove the problematic features from the host model in QEMU, like "we
> just don't support this feature". Consequently, any migration of a VM
> with csske=on to a new QEMU version will fail, similar to having an
> older QEMU version without support for a certain feature.
> 
> "host-passthrough" would change between QEMU versions ... which I see as
> problematic.
> 
> 2. Introduce a new CPU model that has these new semantics: "host model"
> - deprecated features. Migration of older VMs with csske=on to a new
> QEMU version will work. Make libvirt use/expand that new CPU model
> 
> It doesn't necessarily have to be an actual new cpu model. We can use a
> feature group, like "-cpu host,deprectated-features=false". What's
> inside "deprecated-features" will actually change between QEMU versions,
> but we don't really care, as the expanded CPU model won't change.
> 
> "host-passthrough" won't change between QEMU versions ...
> 
> 3. As Daniel suggested, don't use the host model, but a CPU model
> indicated as "suggested".
> 
> The real issue is that in reality, we don't simply always use a model
> like "gen15a", but usually want optional features, if they are around.
> Prime examples are "sie" and friends.
> 
> 
> 
> I tend to prefer 2. With 3. I see issues with optional features like
> "sie" and friends. Often, you really want "give me all you got, but
> disable deprecated features that might cause problems in the future".
> 

David,
if I understand you proposal 2 correctly it sounds a lot like Christians 
idea of leaving the CPU mode "host-model" as is and introduce a new CPU 
mode "host-recommended" for the new semantics in which 
query-cpu-model-expansion would be called with the additional 
"deprectated-features" property.
That way libvirt would not have to fiddle around with the deprecation 
itself and users would have the option which semantic they want to use. 
Is that correct?


-- 
Mit freundlichen Grüßen/Kind regards
    Boris Fiuczynski

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