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

Am 11.03.22 um 15:56 schrieb Daniel P. Berrangé: > On Fri, Mar 11, 2022 at 03:52:57PM +0100, Christian Borntraeger wrote: >> >> >> 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" ? > > Neither of those. Libvirt would simply report this attribute in > the information it exposes about CPUs. > > OpenStack would explicitly extract this and set it in the XML > for the guest, so that each guest's view of "recommended" is > fixed from the time that guest is first created, rather than > potentially changing on each later boots. Openstack is one thing, but I think this flag would really be useful for instantiation without open stack.