iommufd can directly implement the /dev/vfio/vfio container IOCTLs by
mapping them into io_pagetable operations.

A userspace application can test against iommufd and confirm compatibility
then simply make a small change to open /dev/iommu instead of
/dev/vfio/vfio.

For testing purposes /dev/vfio/vfio can be symlinked to /dev/iommu and
then all applications will use the compatibility path with no code
changes. A later series allows /dev/vfio/vfio to be directly provided by
iommufd, which allows the rlimit mode to work the same as well.

This series just provides the iommufd side of compatibility. Actually
linking this to VFIO_SET_CONTAINER is a followup series, with a link in
the cover letter.

Internally the compatibility API uses a normal IOAS object that, like
vfio, is automatically allocated when the first device is
attached.

Userspace can also query or set this IOAS object directly using the
IOMMU_VFIO_IOAS ioctl. This allows mixing and matching new iommufd only
features while still using the VFIO style map/unmap ioctls.

While this is enough to operate qemu, it has a few differences:

 - Resource limits rely on memory cgroups to bound what userspace can do
   instead of the module parameter dma_entry_limit.

 - VFIO P2P is not implemented. The DMABUF patches for vfio are a start at
   a solution where iommufd would import a special DMABUF. This is to avoid
   further propogating the follow_pfn() security problem.

 - A full audit for pedantic compatibility details (eg errnos, etc) has
   not yet been done

 - powerpc SPAPR is left out, as it is not connected to the iommu_domain
   framework. It seems interest in SPAPR is minimal as it is currently
   non-working in v6.1-rc1. They will have to convert to the iommu
   subsystem framework to enjoy iommfd.

The following are not going to be implemented and we expect to remove them
from VFIO type1:

 - SW access 'dirty tracking'. As discussed in the cover letter this will
   be done in VFIO.

 - VFIO_TYPE1_NESTING_IOMMU
    https://lore.kernel.org/all/0-v1-0093c9b0e345+19-vfio_no_nesting_jgg@nvidia.com/

 - VFIO_DMA_MAP_FLAG_VADDR
    https://lore.kernel.org/all/Yz777bJZjTyLrHEQ@nvidia.com/

Link: https://lore.kernel.org/r/15-v6-a196d26f289e+11787-iommufd_jgg@nvidia.comTested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>