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

[David Gibson]

On Tue, Jun 09, 2020 at 08:44:02AM +0200, Cornelia Huck wrote:
> On Mon, 8 Jun 2020 19:00:45 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> 
> > > I'm also not 100% sure about migration... would it make sense to
> > > discuss all of this in the context of the cross-arch patchset? It seems
> > > power has similar issues.
> > >   
> > 
> > I'm going to definitely have a good look at that. What I think special
> > about s390 is that F_ACCESS_PLATFORM is hurting us because all IO needs
> > to go through ZONE_DMA (this is a problem of the implementation that
> > stemming form a limitation of the DMA API, upstream didn't let me
> > fix it). 
> 
> My understanding is that power runs into similar issues, but I don't
> know much about power, so I might be entirely wrong :)

Sort of, but not to the same extent, I think.

[snip]
> > > (Can we disallow transition to pv if it is off? Maybe with the machine
> > > property approach from the cross-arch patchset?)  
> > 
> > No we can't disallow transition because for hotplug that already
> > happened.
> 
> I mean, can a virtio devices without IOMMU_PLATFORM act as a transition
> blocker (i.e. an attempt by a guest to move to pv would fail?)

This might be possible, but I suspect having to explicitly say you
want pv support, then having it validate virtio (and anything else it
needs) will give a better UX.

-- 
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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