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

David Hildenbrand david at redhat.com
Fri Mar 11 09:30:48 UTC 2022

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.

Gluing this to the "host-model" feels wrong.

The other concern I have is that deprecated features are a moving
target, and with a new QEMU version you could suddenly have more
deprecated features. Hm.

Maybe you'd want some kind of a host-based-model from QEMU that does
this automatically? I need more coffee to get creative on a name.


David / dhildenb

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