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

David Hildenbrand david at redhat.com
Mon Mar 21 09:35:23 UTC 2022

On 21.03.22 10:25, Daniel P. Berrangé wrote:
> On Fri, Mar 18, 2022 at 01:23:03PM -0400, Collin Walling wrote:
>> On 3/15/22 15:08, David Hildenbrand wrote:
>>> On 15.03.22 18:40, Boris Fiuczynski wrote:
>>>> 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?
>>> Yes, exactly.
>> From what I understand:
>>  - add a "deprecated-features" feature group (more-or-less David's code)
>> libvirt
>>  - recognize a new model name "host-recommended"
>>  - query QEMU for host-model + deprecated-features and cache it in caps
>> file (something like <hostRecCpu>)
>>  - when guest is defined with "host-recommended", pull <hostRecCPU> from
>> caps when guest is started (similar to how host-model works today)
>> If this is sufficient, then I can then get to work on this.
>> My question is what would be the best way to include the deprecated
>> features when calculating a baseline or comparison. Both work with the
>> host-model and may no longer present an accurate result. Say, for
>> example, we baseline a z15 with a gen17 (which will outright not support
>> CSSKE). With today's implementation, this might result in a ridiculously
>> old CPU model which also does not support CSSKE. The ideal response
>> would be a z15 - deprecated features (i.e. host-recommended on a z15),
>> but we'd need a way to flag to QEMU that we want to exclude the
>> deprecated features. Or am I totally wrong about this?
> QEMU has a concept of versioned QEMU models, so you could define a
> z15-v2 version without CSSKE

gen15a already comes with csske=false. s390x does not implement
versioned CPU models and as I raised in the past, that concept is rather
a bad fit for s390x.


David / dhildenb

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