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

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?