Re: [PATCH RFC 0/1] s390x CPU Model Feature Deprecation

On Fri, Mar 18, 2022 at 01:23:03PM -0400, Collin Walling wrote: > On 3/15/22 15:08, David Hildenbrand wrote: >> On 15.03.22 18:40, Boris Fiuczynski wrote: >>> On 3/15/22 4:58 PM, David Hildenbrand wrote: >>>> On 11.03.22 13:44, Christian Borntraeger wrote: >>>>> >>>>> >>>>> Am 11.03.22 um 10:30 schrieb David Hildenbrand: >>>>>> On 11.03.22 05:17, Collin Walling wrote: >>>>>>> The s390x architecture has a growing list of features that will no longer >>>>>>> be supported on future hardware releases. This introduces an issue with >>>>>>> migration such that guests, running on models with these features enabled, >>>>>>> will be rejected outright by machines that do not support these features. >>>>>>> >>>>>>> A current example is the CSSKE feature that has been deprecated for some time. >>>>>>> It has been publicly announced that gen15 will be the last release to >>>>>>> support this feature, however we have postponed this to gen16a. A possible >>>>>>> solution to remedy this would be to create a new QEMU QMP Response that allows >>>>>>> users to query for deprecated/unsupported features. >>>>>>> >>>>>>> This presents two parts of the puzzle: how to report deprecated features to >>>>>>> a user (libvirt) and how should libvirt handle this information. >>>>>>> >>>>>>> First, let's discuss the latter. The patch presented alongside this cover letter >>>>>>> attempts to solve the migration issue by hard-coding the CSSKE feature to be >>>>>>> disabled for all s390x CPU models. This is done by simply appending the CSSKE >>>>>>> feature with the disabled policy to the host-model. >>>>>>> >>>>>>> libvirt pseudo: >>>>>>> >>>>>>> if arch is s390x >>>>>>> set CSSKE to disabled for host-model >>>>>> >>>>>> That violates host-model semantics and possibly the user intend. There >>>>>> would have to be some toggle to manually specify this, for example, a >>>>>> new model type or a some magical flag. >>>>> >>>>> What we actually want to do is to disable csske completely from QEMU and >>>>> thus from the host-model. Then it would not violate the spec. >>>>> But this has all kind of issues (you cannot migrate from older versions >>>>> of software and machines) although the hardware still can provide the feature. >>>>> >>>>> The hardware guys promised me to deprecate things two generations earlier >>>>> and we usually deprecate things that are not used or where software has a >>>>> runtime switch. >>>>> >>>>> From what I hear from you is that you do not want to modify the host-model >>>>> semantics to something more useful but rather define a new thing (e.g. "host-sane") ? >>>> >>>> My take would be, to keep the host model consistent, meaning, the >>>> semantics in QEMU exactly match the semantics in Libvirt. It defines the >>>> maximum CPU model that's runnable under KVM. If a feature is not >>>> included (e.g., csske) that feature cannot be enabled in any way. >>>> >>>> The "host model" has the semantics of resembling the actual host CPU. >>>> This is only partially true, because we support some features the host >>>> might not support (e.g., zPCI IIRC) and obviously don't support all host >>>> features in QEMU. >>>> >>>> So instead of playing games on the libvirt side with the host model, I >>>> see the following alternatives: >>>> >>>> 1. Remove the problematic features from the host model in QEMU, like "we >>>> just don't support this feature". Consequently, any migration of a VM >>>> with csske=on to a new QEMU version will fail, similar to having an >>>> older QEMU version without support for a certain feature. >>>> >>>> "host-passthrough" would change between QEMU versions ... which I see as >>>> problematic. >>>> >>>> 2. Introduce a new CPU model that has these new semantics: "host model" >>>> - deprecated features. Migration of older VMs with csske=on to a new >>>> QEMU version will work. Make libvirt use/expand that new CPU model >>>> >>>> It doesn't necessarily have to be an actual new cpu model. We can use a >>>> feature group, like "-cpu host,deprectated-features=false". What's >>>> inside "deprecated-features" will actually change between QEMU versions, >>>> but we don't really care, as the expanded CPU model won't change. >>>> >>>> "host-passthrough" won't change between QEMU versions ... >>>> >>>> 3. As Daniel suggested, don't use the host model, but a CPU model >>>> indicated as "suggested". >>>> >>>> The real issue is that in reality, we don't simply always use a model >>>> like "gen15a", but usually want optional features, if they are around. >>>> Prime examples are "sie" and friends. >>>> >>>> >>>> >>>> I tend to prefer 2. With 3. I see issues with optional features like >>>> "sie" and friends. Often, you really want "give me all you got, but >>>> disable deprecated features that might cause problems in the future". >>>> >>> >>> David, >>> if I understand you proposal 2 correctly it sounds a lot like Christians >>> idea of leaving the CPU mode "host-model" as is and introduce a new CPU >>> mode "host-recommended" for the new semantics in which >>> query-cpu-model-expansion would be called with the additional >>> "deprectated-features" property. >>> That way libvirt would not have to fiddle around with the deprecation >>> itself and users would have the option which semantic they want to use. >>> Is that correct? >> >> Yes, exactly. >> >> > > From what I understand: > > QEMU > - add a "deprecated-features" feature group (more-or-less David's code) > > libvirt > - recognize a new model name "host-recommended" > - query QEMU for host-model + deprecated-features and cache it in caps > file (something like <hostRecCpu>) > - when guest is defined with "host-recommended", pull <hostRecCPU> from > caps when guest is started (similar to how host-model works today) > > If this is sufficient, then I can then get to work on this. > > My question is what would be the best way to include the deprecated > features when calculating a baseline or comparison. Both work with the > host-model and may no longer present an accurate result. Say, for > example, we baseline a z15 with a gen17 (which will outright not support > CSSKE). With today's implementation, this might result in a ridiculously > old CPU model which also does not support CSSKE. The ideal response > would be a z15 - deprecated features (i.e. host-recommended on a z15), > but we'd need a way to flag to QEMU that we want to exclude the > deprecated features. Or am I totally wrong about this? QEMU has a concept of versioned QEMU models, so you could define a z15-v2 version without CSSKE