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

Daniel P. Berrangé berrange at redhat.com
Fri Mar 11 12:37:46 UTC 2022

On Fri, Mar 11, 2022 at 01:12:35PM +0100, Christian Borntraeger wrote:
> 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.

This proposal wouldn't have helped in the case of Intel removing
TSX, because it was removed without prior warning in the middle
of the product lifecycle. At that time there were already millions
of VMs in existance using the removed feature.

> > > 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 change you proposal works functionally, but none the less it is
changing the semantics of host-model. It is defined to expose all the
features in the host, and the proposal changes yet. If an app actually
/wants/ to use the deprecated feature and it exists in the host, then
host-model should be allowing that as it does today.

The problem scenario you describe is ultimately that OpenStack does
not have a future proof default CPU choice. Libvirt and QEMU provide
a mechanism for them to pick other CPU models that would address the
problem, but they're not using that. The challenge is that OpenStack
defaults currently are a zero-interaction thing.

They could retain their zero-interaction defaults, if at install time
they queried the libvirt capabilities to learn which named CPU models
are available, whereupon they could decide to use gen15a.  The main
challenge here is that the list of named CPU models is an unordered
set, so it is hard to programatically figure out which of the available
named CPU models is the newest/best/recommended.

IOW, what's missing is a way for apps to easily identify that 'gen15a'
is the best CPU to use on the host, without needing human interaction.

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