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    SubjectRe: [PATCH 1/2] Isolation groups
    On Tue, Mar 27, 2012 at 01:34:43PM -0600, Alex Williamson wrote:
    > > > > 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 | minimalist, thank you. NOT _the_ _other_
    | _way_ _around_!

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