Re: [Qemu-devel] [PATCH 1/2] Isolation groups

[David Gibson <david@gibson.dropbear.id.au>]

On Tue, Mar 27, 2012 at 01:34:43PM -0600, Alex Williamson wrote:
[snip]
> > > > this case, it gets a bit complex.  When the FooBus isolation provider
> > > > is active, the FooBus devices would be in their own groups, not the
> > > > group of the FooBridge and its sibling.  When the FooBus isolation
> > > > provider is removed, it would have to configure the FooBus IOMMU to a
> > > > passthrough mode, and revert the FooBus devices to the parent's
> > > > group.  Hm.  Complicated.
> > > 
> > > Yep.  I think we're arriving at the same point.  Groups are
> > > hierarchical, but ownership via a manager cannot be nested.  So to
> > > manage a group, we need to walk the entire tree of devices below each
> > > device checking that none of the groups are managed and all the devices
> > > are using the right driver, then walk up from the group to verify no
> > > group of a parent device is managed.  Thanks,
> > 
> > Blargh.  I really, really hope we can come up with a simpler model
> > than that.
> 
> Yep, I'm pretty well at the end of this experiment.  Honestly, I think
> isolation groups are the wrong approach.  We're trying to wrap too many
> concepts together and it's completely unmanageable.  I cannot see adding
> the complexity we're talking about here to the core device model for
> such a niche usage.  I think we're better off going back to the
> iommu_device_group() and building that out into something more complete,
> starting with group based iommu ops and a dma quirk infrastructure.
> >From there we can add some basic facilities to toggle driver autoprobe,
> maybe setup notifications for the group, and hopefully include a way to
> share iommu mappings between groups.  Anything much beyond that we
> should probably leave for something like the vfio driver.  Thanks,

Yes, well, I was hoping for a simpler model that didn't involve simply
sweeping all the issues under a rug.

-- 
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