Re: [PATCH] vfio: Remove VFIO_TYPE1_NESTING_IOMMU

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, May 10, 2022 at 01:55:24PM -0300, Jason Gunthorpe wrote:
> This control causes the ARM SMMU drivers to choose a stage 2
> implementation for the IO pagetable (vs the stage 1 usual default),
> however this choice has no visible impact to the VFIO user. Further qemu
> never implemented this and no other userspace user is known.
> 
> The original description in commit f5c9ecebaf2a ("vfio/iommu_type1: add
> new VFIO_TYPE1_NESTING_IOMMU IOMMU type") suggested this was to "provide
> SMMU translation services to the guest operating system" however the rest
> of the API to set the guest table pointer for the stage 1 was never
> completed, or at least never upstreamed, rendering this part useless dead
> code.
> 
> Since the current patches to enable nested translation, aka userspace page
> tables, rely on iommufd and will not use the enable_nesting()
> iommu_domain_op, remove this infrastructure. However, don't cut too deep
> into the SMMU drivers for now expecting the iommufd work to pick it up -
> we still need to create S2 IO page tables.
> 
> Remove VFIO_TYPE1_NESTING_IOMMU and everything under it including the
> enable_nesting iommu_domain_op.
> 
> Just in-case there is some userspace using this continue to treat
> requesting it as a NOP, but do not advertise support any more.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Sanity-tested with qemu-system-aarch64 using "iommu=nested-smmuv3"
(unmerged feature) and "iommu=none" strings on top of vfio next.

Tested-by: Nicolin Chen <nicolinc@nvidia.com>