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

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: