[PATCH RFC 1/1] qemu: capabilities: disable csske for host cpu

[Christian Borntraeger]

Am 11.03.22 um 10:23 schrieb David Hildenbrand:
> On 11.03.22 10:17, Daniel P. Berrangé wrote:
>> On Thu, Mar 10, 2022 at 11:17:38PM -0500, Collin Walling wrote:
>>> CPU models past gen16a will no longer support the csske feature. In
>>> order to secure migration of guests running on machines that still
>>> support this feature to machines that do not, let's disable csske
>>> in the host-model.
> 
> Sorry to say, removing CPU features is a no-go when wanting to guarantee
> forward migration without taking care about CPU model details manually
> and simply using the host model. Self-made HW vendor problem.

And this simply does not reflect reality. Intel and Power have removed TX
for example. We can now sit back and please ourselves how we live in our
world of dreams. Or we can try to define an interface that deals with
reality and actually solves problems.

> 
>>
>> The problem scenario you describe is the intended semantics of
>> host-model though. It enables all features available in the host
>> that you launched on. It lets you live migrate to a target host
>> with the same, or a greater number of features. If the target has
>> a greater number of features, it should restrict the VM to the
>> subset of features that were present on the original source CPU.
>> If the target has fewer features, then you simply can't live
>> migrate a VM using host-model.
>>
>> To get live migration in both directions across CPUs with differing
>> featuresets, then the VM needs to be configured with a named CPU
>> model that is a subset of both, rather than host-model.
> 
> Right, and cpu-model-baseline does that job for you if you're lazy to
> lookup the proper model.

Yes baseline will work, but this requires tooling like openstack. The normal
user will just use the default and this is host-model.

Let me explain the usecase for this feature. Migration between different versins
baseline: always works
host-passthrough: you get what you deserve
default model: works
We have disabled CSSKE from our default models (-cpu gen15a will not present csske).
So that works as well.
host-model: Also works for all machines that have csske.
Now: Lets say gen17 will no longer support this. That means that we can not migrate
host-model from gen16 or gen15 because those will have csske.
What options do we have? If we disable csske in the host capabilities that would mean
that a host compare against an xml from an older QEMU would fail (even if you move
from gen14 to gen14). So this is not a good option.

By disabling deprecated features ONLY for the _initial_ expansion of model-model, but
keeping it in the host capabilities you can migrate existing guests (with the
feature) as we only disable in the expansion, but manually asking for it still works.
AND it will allow to move this instantiation of the guest to future machines without
the feature. Basically everything works.

The alternative of removing csske would result in too many failure scenarios.