question about iommu_need_mapping

From: Jerry Snitselaar
Date: Wed Feb 19 2020 - 18:56:23 EST

Is it possible for a device to end up with dev->archdata.iommu == NULL
on iommu_need_mapping in the following instance:

1. iommu_group has dma domain for default
2. device gets private identity domain in intel_iommu_add_device
3. iommu_need_mapping gets called with that device.
4. dmar_remove_one_dev_info sets dev->archdata.iommu = NULL via unlink_domain_info.
5. request_default_domain_for_dev exits after checking that group->default_domain
exists, and group->default_domain->type is dma.
6. iommu_request_dma_domain_for_dev returns 0 from request_default_domain_for_dev
and a private dma domain isn't created for the device.

The case I was seeing went away with commit 9235cb13d7d1 ("iommu/vt-d:
Allow devices with RMRRs to use identity domain"), because it changed
which domain the group and devices were using, but it seems like it is
still a possibility with the code. Baolu, you mentioned possibly
removing the domain switch. Commit 98b2fffb5e27 ("iommu/vt-d: Handle
32bit device with identity default domain") makes it sound like the
domain switch is required.