An iommufd fault object provides an interface for delivering I/O page
faults to user space. These objects are created and destroyed by user
space, and they can be associated with or dissociated from hardware page
table objects during page table allocation or destruction.

User space interacts with the fault object through a file interface. This
interface offers a straightforward and efficient way for user space to
handle page faults. It allows user space to read fault messages
sequentially and respond to them by writing to the same file. The file
interface supports reading messages in poll mode, so it's recommended that
user space applications use io_uring to enhance read and write efficiency.

A fault object can be associated with any iopf-capable iommufd_hw_pgtable
during the pgtable's allocation. All I/O page faults triggered by devices
when accessing the I/O addresses of an iommufd_hw_pgtable are routed
through the fault object to user space. Similarly, user space's responses
to these page faults are routed back to the iommu device driver through
the same fault object.

Link: https://lore.kernel.org/r/20240702063444.105814-7-baolu.lu@linux.intel.comSigned-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>