On Fri, Jun 29, 2018 at 09:53:53AM +0100, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (ehabkost@xxxxxxxxxx) 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.
True, but if more vulns arrive we have the same problem with named
suffixes too. eg if we added -SSBD variants, users would ask whether
-SSBD includes the -IBRS fix or vica-verca, as a year down the line
they're not going to remember which or SSBD/IBRS came out first.
> 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.
The kernel has a /sys/devices/system/cpu/vulnerabilities dir
that lists status of various flaws.
I have been thinking about whether libvirt should create a
'virt-guest-validate' command that looks at guest XML and
reports whether any of the config settings are vulnerable
or otherwise diverging from best practice in some way.
QEMU itself would perhaps have a 'query-vulnerabilities'
monitor command to report whether the current config is
satisfactory or not.
Ultimately though, getting a fixed guest involves host
kernel, microcode, qemu, and guest kernel. So to get a
true picture of your safety people should really look
straight to the guest kernels' /sys/devices/system/cpu/vulnerabilities
directory. They only need to look at host/microcode/qemu if the
guest is reporting something is wrong.
Regards,
Daniel
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