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Date:   Mon, 14 Feb 2022 08:45:18 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        Alex Williamson <alex.williamson@redhat.com>,
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        Cornelia Huck <cohuck@redhat.com>,
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        iommu@lists.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@intel.com>,
        Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()
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On Mon, Feb 14, 2022 at 12:09:36PM +0000, Robin Murphy wrote:
> On 2022-01-06 02:20, Lu Baolu wrote:
> > Expose an interface to replace the domain of an iommu group for frameworks
> > like vfio which claims the ownership of the whole iommu group.
> 
> But if the underlying point is the new expectation that
> iommu_{attach,detach}_device() operate on the device's whole group where
> relevant, why should we invent some special mechanism for VFIO to be
> needlessly inconsistent?
> 
> I said before that it's trivial for VFIO to resolve a suitable device if it
> needs to; by now I've actually written the patch ;)
> 
> https://gitlab.arm.com/linux-arm/linux-rm/-/commit/9f37d8c17c9b606abc96e1f1001c0b97c8b93ed5

Er, how does locking work there? What keeps busdev from being
concurrently unplugged? How can iommu_group_get() be safely called on
this pointer?

All of the above only works normally inside a probe/remove context
where the driver core is blocking concurrent unplug and descruction.

I think I said this last time you brought it up that lifetime was the
challenge with this idea.

Jason