vfio: Remove VFIO_TYPE1_NESTING_IOMMU

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 significant visible impact to the VFIO
user. Further qemu never implemented this and no other userspace user is
known.

The original description in commit f5c9ecebaf ("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 and manage
invalidation was never completed, or at least never upstreamed, rendering
this part useless dead code.

Upstream has now settled on iommufd as the uAPI for controlling nested
translation. Choosing the stage 2 implementation should be done by through
the IOMMU_HWPT_ALLOC_NEST_PARENT flag during domain allocation.

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.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Donald Dutile <ddutile@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v4-9e99b76f3518+3a8-smmuv3_nesting_jgg@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>

include/uapi/linux/vfio.h

@@ -35,7 +35,7 @@
 #define VFIO_EEH			5
 
 /* Two-stage IOMMU */
-#define VFIO_TYPE1_NESTING_IOMMU	6	/* Implies v2 */
+#define __VFIO_RESERVED_TYPE1_NESTING_IOMMU	6	/* Implies v2 */
 
 #define VFIO_SPAPR_TCE_v2_IOMMU		7