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

[Daniel P. Berrangé]

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.

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.

With regards,
Daniel
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