Re: Upstream QEMU guest support policy ? Re: [PATCH v3 0/2] spapr: Use vIOMMU translation for virtio by default

[David Gibson]

On Wed, Mar 11, 2020 at 07:48:26AM -0400, Michael S. Tsirkin wrote:
> On Wed, Mar 11, 2020 at 10:01:27AM +0000, Daniel P. Berrangé wrote:
> > On Wed, Mar 11, 2020 at 12:12:47PM +1100, David Gibson wrote:
> > > On Tue, Mar 10, 2020 at 11:43:43AM +0000, Daniel P. Berrangé wrote:
> > > > On Thu, Mar 05, 2020 at 03:30:07PM +1100, David Gibson wrote:
> > > > > Upcoming Secure VM support for pSeries machines introduces some
> > > > > complications for virtio, since the transfer buffers need to be
> > > > > explicitly shared so that the hypervisor can access them.
> > > > > 
> > > > > While it's not strictly speaking dependent on it, the fact that virtio
> > > > > devices bypass normal platform IOMMU translation complicates the issue
> > > > > on the guest side.  Since there are some significan downsides to
> > > > > bypassing the vIOMMU anyway, let's just disable that.
> > > > > 
> > > > > There's already a flag to do this in virtio, just turn it on by
> > > > > default for forthcoming pseries machine types.
> > > > 
> > > > Breaking existing guest OS to support a new secure VM feature that
> > > > may not even be used/wanted doesn't seems like a sensible tradeoff
> > > > for default out of the box behaviour.
> > > > 
> > > > IOW, if Secure VM needs this, can we tie the change in virtio and
> > > > IOMMU defaults to the machine type flag that enables the use of
> > > > Secure VM.
> > > 
> > > There is no such flag.
> > > 
> > > In the POWER secure VM model, the secure mode option isn't something
> > > that's constructed in when the hypervisor builds the VM.  Instead the
> > > VM is started normally and transitions itself to secure mode by
> > > talking directly with the ultravisor (it then uses TPM shenannigans to
> > > safely get the keys to its real storage backend(s)).
> > 
> > This is pretty suprising to me. The ability to use secure VM mode surely
> > depends on host hardware features. We would need to be able to block the
> > use of this, in order to allow VMs to be live migrated to hosts which
> > lack the feature. Automatically & silently enabling a feature that
> > has a hardware dependancy is something we aim to avoid, unless the user
> > has opted in via some flag (such as -cpu host, or a -cpu $NAME, that
> > implies the feature).
> 
> That's something I don't know. Is migration supported in this mode?

Not at this stage, though there's plans for it later.

-- 
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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