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

Fri Mar 11 15:43:01 UTC 2022

On Fri, Mar 11, 2022 at 04:24:22PM +0100, Christian Borntraeger wrote:
> 
> 
> 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.

Sure, any mgmt app using libvirt that provisions guests can use
this approach. I just mentioned openstack as that was what you
mentioned at the start of this thread.

Regards,
Daniel
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