Re: [PATCH v3 9/9] host trust limitation: Alter virtio default properties for protected guests

[Halil Pasic <pasic@linux.ibm.com>]

On Fri, 19 Jun 2020 13:16:38 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

[..]

> > > > > @@ -1165,6 +1167,15 @@ void machine_run_board_init(MachineState *machine)
> > > > >           * areas.
> > > > >           */
> > > > >          machine_set_mem_merge(OBJECT(machine), false, &error_abort);
> > > > > +
> > > > > +        /*
> > > > > +         * Virtio devices can't count on directly accessing guest
> > > > > +         * memory, so they need iommu_platform=on to use normal DMA
> > > > > +         * mechanisms.  That requires disabling legacy virtio support
> > > > > +         * for virtio pci devices
> > > > > +         */
> > > > > +        object_register_sugar_prop(TYPE_VIRTIO_PCI, "disable-legacy", "on");
> > > > > +        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform", "on");
> > > > >      }  
> > > > 
> > > > Silently changing the user's request configuration like this is a bad idea.
> > > > The "disable-legacy" option in particular is undesirable as that switches
> > > > the device to virtio-1.0 only mode, which exposes a different PCI ID to
> > > > the guest.
> > > > 
> > > > If some options are incompatible with encryption, then we should raise a
> > > > fatal error at startup, so applications/admins are aware that their requested
> > > > config is broken.  
> > >
> > > Agreed - my suggestion is an on/off/auto property, auto value
> > > changes automatically, on/off is validated.  
> > 
> > In fact should we extend all bit properties to allow an auto value?  
> 
> If "auto" was made the default that creates a similar headache, as to
> preserve existing configuration semantics we expose to apps, libvirt
> would need to find all the properties changed to use "auto" and manually
> set them back to on/off explicitly.

Hm, does that mean we can't change the default for any qemu property?

I would expect that the defaults most remain invariant for any
particular machine version. Conceptually, IMHO the default could change
with a new machine version, but we would need some mechanism to ensure
compatibility for old machine versions.

Regards,
Halil