Re: [Qemu-devel] CPU model versioning separate from machine type versioning ?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> 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:
> 

Both of those are solved with the numbering scheme

> * 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".

Use the highest that works.

> * Ordering/preference info, e.g.: "Haswell-3.1 is better than
>   Haswell-3.0, prefer the latter"

Higher is better.

The only thing that worries me about a numbering scheme is that
it's now more difficult for a user to know whether they've got
the type with a fix for a particular vulnerability.
We're going to have to say something like:
  'For the new XYZ vulnerability make sure you're using
  Haswell-3.2 or later, SkyLake-2.6 or later, Westmere-4.8 or later
  .....'

which all gets a bit confusing.

Dave


> -- 
> Eduardo
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK