Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

[Jason Gunthorpe]

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