When a PASID is used for SVA by the device, it's possible that the PASID
entry is cleared before the device flushes all ongoing DMA requests. The
IOMMU should tolerate and ignore the non-recoverable faults caused by the
untranslated requests from this device.

For example, when an exception happens, the process terminates before the
device driver stops DMA and call IOMMU driver to unbind PASID. The flow
of process exist is as follows:

do_exit() {
     exit_mm() {
             mm_put();
             exit_mmap() {
                     intel_invalidate_range() //mmu notifier
                     tlb_finish_mmu()
                     mmu_notifier_release(mm) {
                             intel_iommu_release() {
[2]                                  intel_iommu_teardown_pasid();
                                     intel_iommu_flush_tlbs();
                             }
                     }
                     unmap_vmas();
                     free_pgtables();
             };
     }
     exit_files(tsk) {
             close_files() {
                     dsa_close();
[1]                  dsa_stop_dma();
                     intel_svm_unbind_pasid();
             }
     }
}

Care must be taken on VT-d to avoid unrecoverable faults between the time
window of [1] and [2]. [Process exist flow was contributed by Jacob Pan.]

Intel VT-d provides such function through the FPD bit of the PASID entry.
This sets FPD bit when PASID entry is changing from present to nonpresent
in the mm notifier and will clear it when the pasid is unbound.
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Link: https://lore.kernel.org/r/20200516062101.29541-15-baolu.lu@linux.intel.comSigned-off-by: Joerg Roedel <jroedel@suse.de>