Re: [PATCH v3 0/9] Generalize memory encryption models

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jun 26, 2020 at 11:29:03AM +0100, Dr. David Alan Gilbert wrote:
> * Janosch Frank (frankja@linux.ibm.com) wrote:
> > On 6/26/20 11:32 AM, Daniel P. Berrangé wrote:
> > > On Fri, Jun 26, 2020 at 11:01:58AM +0200, Janosch Frank wrote:
> > >> On 6/26/20 8:53 AM, David Hildenbrand wrote:
> > >>>>>>> Does this have any implications when probing with the 'none' machine?
> > >>>>>>
> > >>>>>> I'm not sure.  In your case, I guess the cpu bit would still show up
> > >>>>>> as before, so it would tell you base feature availability, but not
> > >>>>>> whether you can use the new configuration option.
> > >>>>>>
> > >>>>>> Since the HTL option is generic, you could still set it on the "none"
> > >>>>>> machine, though it wouldn't really have any effect.  That is, if you
> > >>>>>> could create a suitable object to point it at, which would depend on
> > >>>>>> ... details.
> > >>>>>>
> > >>>>>
> > >>>>> The important point is that we never want the (expanded) host cpu model
> > >>>>> look different when either specifying or not specifying the HTL
> > >>>>> property.
> > >>>>
> > >>>> Ah, yes, I see your point.  So my current suggestion will satisfy
> > >>>> that, basically it is:
> > >>>>
> > >>>> cpu has unpack (inc. by default) && htl specified
> > >>>> 	=> works (allowing secure), as expected
> > >>>
> > >>> ack
> > >>>
> > >>>>
> > >>>> !cpu has unpack && htl specified
> > >>>> 	=> bails out with an error
> > >>>
> > >>> ack
> > >>>
> > >>>>
> > >>>> !cpu has unpack && !htl specified
> > >>>> 	=> works for a non-secure guest, as expected
> > >>>> 	=> guest will fail if it attempts to go secure
> > >>>
> > >>> ack, behavior just like running on older hw without unpack
> > >>>
> > >>>>
> > >>>> cpu has unpack && !htl specified
> > >>>> 	=> works as expected for a non-secure guest (unpack feature is
> > >>>> 	   present, but unused)
> > >>>> 	=> secure guest may work "by accident", but only if all virtio
> > >>>> 	   properties have the right values, which is the user's
> > >>>> 	   problem
> > >>>>
> > >>>> That last case is kinda ugly, but I think it's tolerable.
> > >>>
> > >>> Right, we must not affect non-secure guests, and existing secure setups
> > >>> (e.g., older qemu machines). Will have to think about this some more,
> > >>> but does not sound too crazy.
> > >>
> > >> I severely dislike having to specify things to make PV work.
> > >> The IOMMU is already a thorn in our side and we're working on making the
> > >> whole ordeal completely transparent so the only requirement to make this
> > >> work is the right machine, kernel, qemu and kernel cmd line option
> > >> "prot_virt=1". That's why we do the reboot into PV mode in the first place.
> > >>
> > >> I.e. the goal is that if customers convert compatible guests into
> > >> protected ones and start them up on a z15 on a distro with PV support
> > >> they can just use the guest without having to change XML or command line
> > >> parameters.
> > > 
> > > If you're exposing new features to the guest machine, then it is usually
> > > to be expected that XML and QEMU command line will change. Some simple
> > > things might be hidable behind a new QEMU machine type or CPU model, but
> > > there's a limit to how much should be hidden that way while staying sane.
> > > 
> > > I'd really expect the configuration to change when switching a guest to
> > > a new hardware platform and wanting major new functionality to be enabled.
> > > The XML / QEMU config is a low level instantiation of a particular feature
> > > set, optimized for a specific machine, rather than a high level description
> > > of ideal "best" config independent of host machine.
> > 
> > You still have to set the host command line and make sure that unpack is
> > available. Currently you also have to specify the IOMMU which we like to
> > drop as a requirement. Everything else is dependent on runtime
> > information which tells us if we need to take a PV or non-PV branch.
> > Having the unpack facility should be enough to use the unpack facility.
> > 
> > Keep in mind that we have no real concept of a special protected VM to
> > begin with. If the VM never boots into a protected kernel it will never
> > be protected. On a reboot it drops from protected into unprotected mode
> > to execute the bios and boot loader and then may or may not move back
> > into a protected state.
> 
> My worry isn't actually how painful adding all the iommu glue is, but
> what happens when users forget; especially if they forget for one
> device.
> 
> I could appreciate having a machine option to cause iommu to then get
> turned on with all other devices; but I think also we could do with
> something that failed with a nice error if an iommu flag was missing.
> For SEV this could be done pretty early, but for power/s390 I guess
> you'd have to do this when someone tried to enable secure mode, but
> I'm not sure you can tell.

What is the cost / downside of turning on the iommu option for virtio
devices ? Is it something that is reasonable for a mgmt app todo
unconditionally, regardless of whether memory encryption is in use,
or will that have a negative impact on things ?

Regards,
Daniel
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