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. -- Thanks, David / dhildenb