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

[Cornelia Huck]

On Thu, 25 Jun 2020 08:59:00 +0200
David Hildenbrand <david@redhat.com> wrote:

> >>>> How do upper layers actually figure out if memory encryption etc is
> >>>> available? on s390x, it's simply via the expanded host CPU model.  
> >>>
> >>> Haven't really tackled that yet.  But one way that works for multiple
> >>> systems has got to be better than a separate one for each, right?  
> >>
> >> I think that's an important piece. Especially once multiple different
> >> approaches are theoretically available one wants to sense from upper 
> >> layers.  
> > 
> > Fair point.
> > 
> > So... IIRC there's a general way of looking at available properties
> > for any object, including the machine.  So we can probe for
> > availability of the "host-trust-limitation" property itself easily
> > enough.  
> 
> You can have a look at how it's currently probed by libvirt in
> 
> https://www.redhat.com/archives/libvir-list/2020-June/msg00518.html
> 
> For now, the s390x check consists of
> - checking if /sys/firmware/uv is available
> - checking if the kernel cmdline contains 'prot_virt=1'
> 
> The sev check is
> - checking if /sys/module/kvm_amd/parameters/sev contains the
>    value '1'
> - checking if /dev/sev
> 
> So at least libvirt does not sense via the CPU model on s390x yet.

It checks for 158 (which is apparently 'host supports secure
execution'). IIUC, only 161 ('unpack facility') is relevant for the
guest... does that also show up on the host? (I guess it does, as it
describes an ultravisor feature, IIUC.) If it is always implied,
libvirt probably does not need an extra check.