Re: [libvirt] CPU model versioning separate from machine type versioning ?

On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...] > What if we can borrow the concept of versioning from machine types and apply > it to CPU models directly. For example, considering the history of "Haswell" > in QEMU, if we had versioned things, we would by now have: > > Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) > Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ > Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) > Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 > Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) > Haswell-3.0.0 - added 'ssbd' (never done) > > If we followed the machine type approach, then a bare "Haswell" would > statically resolve at build time to the most recent Haswell-X.X.X version > associated with the QEMU release. This is unhelpful as we have a direct > dependancy on the host hardware features. Better would be for a bare > "Haswell" to be dynamically resolved at runtime, picking the most recent > version that is capable of launching given the current hardware, KVM/TCG impl > and QEMU version. > > ie -cpu Haswell > > should use Haswell-2.5.0 if on silicon with the TSX errata applied, > but use Haswell-2.12.0 if the Spectre errata is applied in microcode, > and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata. Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.

> Versioning of CPU models as opposed to using arbitrary string suffixes > (-noTSX, -IBRS) has a number of usability improvements that we would > gain with versioned machine types, while avoiding exploding the machine > type matrix. With versioned CPU models we can > > - Automatically tailor the best model based on hardware support > > - Users always get the best model if they use the bare CPU name > > - It is obvious to users which is the "best" / "newest" CPU model > > - Avoid combinatorial expansion of machines since same CPU model > version can be added to all releases without adding machine types. > > - Users can still force a specific downgraded model by using the > fully versioned name. > > Such versioning of CPU models would largely "just work" with existing > libvirt versions, but to libvirt would really want to expand the bare > CPU name to a versioned CPU name when recording new guest XML, so the > ABI is preserved long term. > > An application like virt-manager which wants a simple UI can forever be > happy simply giving users a list of bare CPU model names, and allowing > libvirt / QEMU to automatically expand to the best versioned model for > their host. > > An application like oVirt/OpenStack which wants direct control can allow > the admin to choice if a bare name, or explicitly picking a versioned name > if they need to cope with possibility of outdated hosts. > The proposal makes sense, and I think most of it can be already implemented on top of existing query-cpu-model-* commands. query-cpu-model-expansion type=static can expand to a versioned CPU model. We will probably need to make query-cpu-model-expansion accept a machine-type name as input, and/or add a new flag meaning "please give me the best CPU version you have, not the one defined by the current machine-type". I'm not sure what would be the best way to encode two types of information, though: * Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell". * Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter"