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 With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
