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

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?