Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV

[David Gibson]

On Tue, Jun 09, 2020 at 12:44:39PM -0400, Michael S. Tsirkin wrote:
> On Tue, Jun 09, 2020 at 06:28:39PM +0200, Halil Pasic wrote:
> > On Tue, 9 Jun 2020 17:47:47 +0200
> > Claudio Imbrenda <imbrenda@linux.ibm.com> wrote:
> > 
> > > On Tue, 9 Jun 2020 11:41:30 +0200
> > > Halil Pasic <pasic@linux.ibm.com> wrote:
> > > 
> > > [...]
> > > 
> > > > I don't know. Janosch could answer that, but he is on vacation. Adding
> > > > Claudio maybe he can answer. My understanding is, that while it might
> > > > be possible, it is ugly at best. The ability to do a transition is
> > > > indicated by a CPU model feature. Indicating the feature to the guest
> > > > and then failing the transition sounds wrong to me.
> > > 
> > > I agree. If the feature is advertised, then it has to work. I don't
> > > think we even have an architected way to fail the transition for that
> > > reason.
> > > 
> > > What __could__ be done is to prevent qemu from even starting if an
> > > incompatible device is specified together with PV.
> > 
> > AFAIU, the "specified together with PV" is the problem here. Currently
> > we don't "specify PV" but PV is just a capability that is managed by the
> > CPU model (like so many other).
> 
> So if we want to keep it user friendly, there could be
> protection property with values on/off/auto, and auto
> would poke at host capability to figure out whether
> it's supported.
> 
> Both virtio and CPU would inherit from that.

Right, that's what I have in mind for my 'host-trust-limitation'
property (a generalized version of the existing 'memory-encryption'
machine option).  My draft patches already set virtio properties
accordingly, it should be possible to set (default) cpu properties as
well.

> This will allow other useful features such as ability
> to hide PV from guest, which could in turn be handy e.g.
> to allow migration to hosts without PV support,
> or if host wants to force ability to read guest memory
> e.g. for security.
> 
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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