iommu: iommu_group_claim_dma_owner() must always assign a domain

Once the group enters 'owned' mode it can never be assigned back to the
default_domain or to a NULL domain. It must always be actively assigned to
a current domain. If the caller hasn't provided a domain then the core
must provide an explicit DMA blocking domain that has no DMA map.

Lazily create a group-global blocking DMA domain when
iommu_group_claim_dma_owner is first called and immediately assign the
group to it. This ensures that DMA is immediately fully isolated on all
IOMMU drivers.

If the user attaches/detaches while owned then detach will set the group
back to the blocking domain.

Slightly reorganize the call chains so that
__iommu_group_set_core_domain() is the function that removes any caller
configured domain and sets the domains back a core owned domain with an
appropriate lifetime.

__iommu_group_set_domain() is the worker function that can change the
domain assigned to a group to any target domain, including NULL.

Add comments clarifying how the NULL vs detach_dev vs default_domain works
based on Robin's remarks.

This fixes an oops with VFIO and SMMUv3 because VFIO will call
iommu_detach_group() and then immediately iommu_domain_free(), but
SMMUv3 has no way to know that the domain it is holding a pointer to
has been freed. Now the iommu_detach_group() will assign the blocking
domain and SMMUv3 will no longer hold a stale domain reference.

Fixes: 1ea2a07a

 ("iommu: Add DMA ownership management interfaces")
Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Tested-by: Baolu Lu <baolu.lu@linux.intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Co-developed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
--

Just minor polishing as discussed

v3:
 - Change names to __iommu_group_set_domain() /
   __iommu_group_set_core_domain()
 - Clarify comments
 - Call __iommu_group_set_domain() directly in
   iommu_group_release_dma_owner() since we know it is always selecting
   the default_domain
 - Remove redundant detach_dev ops check in __iommu_detach_device and
   make the added WARN_ON fail instead
 - Check for blocking_domain in __iommu_attach_group() so VFIO can
   actually attach a new group
 - Update comments and spelling
 - Fix missed change to new_domain in iommu_group_do_detach_device()

v2: https://lore.kernel.org/r/0-v2-f62259511ac0+6-iommu_dma_block_jgg@nvidia.com
v1: https://lore.kernel.org/r/0-v1-6e9d2d0a759d+11b-iommu_dma_block_jgg@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/0-v3-db7f0785022b+149-iommu_dma_block_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>