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

[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.

> 
> 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.

-- 
Thanks,

David / dhildenb