On Thu, Jun 23, 2022 at 01:00:27PM -0700, Nicolin Chen wrote:
> The domain->ops validation was added, as a precaution, for mixed-driver
> systems.
> Per Robin's remarks,
> * While bus_set_iommu() still exists, the core code prevents multiple
>   drivers from registering, so we can't really run into a situation of
>   having a mixed-driver system:
>   https://lore.kernel.org/kvm/6e1280c5-4b22-ebb3-3912-6c72bc169...@arm.com/
> * And there's plenty more significant problems than this to fix; in future
>   when many can be permitted, we will rely on the IOMMU core code to check
>   the domain->ops:
>   https://lore.kernel.org/kvm/6575de6d-94ba-c427-5b1e-967750ddf...@arm.com/
> So remove the check in VFIO for simplicity.

As discussed we can't remove this check, multiple iommus on different
busses are allowed today and VFIO does not restrict its attempts to
attach a pre-existing domain to a single group or bus.

Please go back to the original version with the ops check in the core

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