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

Am 11.03.22 um 14:08 schrieb Daniel P. Berrangé: > On Fri, Mar 11, 2022 at 12:37:46PM +0000, Daniel P. Berrangé wrote: > > 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. > > I think this could be solved with a change to query-cpu-definitions > in QEMU, to add an extra 'recommended: bool' attribute to the > CpuDefinitionInfo struct. This would be defined to be only set for > 1 CPU model in the list, and would reflect the recommended CPU model > given the current version of QEMU, kernel and hardware. Or we could > allow 'recommended' to be set for more than 1 CPU, provided we define > an explicit ordering of returned CPU models. I like the recommended: bool attribute. It should provide what we need. Would you then also suggest to use this for host-model or only for a new type like "host-recommended" ?