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

Collin Walling walling at linux.ibm.com
Fri Mar 18 17:23:03 UTC 2022

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)

 - 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?


Stay safe and stay healthy

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