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

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.

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.