VFIO's unsafe_noiommu_mode has long provided a way for userspace drivers
to operate on platforms lacking a hardware IOMMU. Today, IOMMUFD also
supports No-IOMMU mode for group-based devices under vfio_compat mode.
However, IOMMUFD's native character device (cdev) does not yet support
No-IOMMU mode, which is the purpose of this patch.

In summary, we have:

|-------------------------+------+---------------|
| Device access mode      | VFIO | IOMMUFD       |
|-------------------------+------+---------------|
| group /dev/vfio/$GROUP  | Yes  | Yes           |
|-------------------------+------+---------------|
| cdev /dev/vfio/devices/ | No   | This patch    |
|-------------------------+------+---------------|

Beyond enabling cdev for IOMMUFD, this patch also addresses the following
deficiencies in the current No-IOMMU mode suggested by Jason[1]:
- Devices operating under No-IOMMU mode are limited to device-level UAPI
  access, without container or IOAS-level capabilities. Consequently,
  user-space drivers lack structured mechanisms for page pinning and often
  resort to mlock(), which is less robust than pin_user_pages() used for
  devices backed by a physical IOMMU. For example, mlock() does not prevent
  page migration.
- There is no architectural mechanism for obtaining physical addresses for
  DMA. As a workaround, user-space drivers frequently rely on /proc/pagemap
  tricks or hardcoded values.

By allowing noiommu device access to IOMMUFD IOAS and HWPT objects, this
patch brings No-IOMMU mode closer to full citizenship within the IOMMU
subsystem. In addition to addressing the two deficiencies mentioned above,
the expectation is that it will also enable No-IOMMU devices to seamlessly
participate in live update sessions via KHO [2].

Furthermore, these devices will use the IOMMUFD-based ownership checking model for
VFIO_DEVICE_PCI_HOT_RESET, eliminating the need for an iommufd_access object
as required in a previous attempt [3].

ChangeLog:
V2:
- Fix build depenency by adding IOMMU_SUPPORT in [8/11]
- Add an optimization to scan beyond the first page for a contiguous physical
  address range and return its length instead of a single page.[4/11]

Since RFC[4]:
- Abandoned dummy iommu driver approach as patch 1-3 absorbed the
  changes into iommufd.

[1] https://lore.kernel.org/linux-iommu/20250603175403.GA407344@nvidia.com/
[2] https://lore.kernel.org/linux-pci/20251027134430.00007e46@linux.microsoft.com/
[3] https://lore.kernel.org/kvm/20230522115751.326947-1-yi.l.liu@intel.com/
[4] https://lore.kernel.org/linux-iommu/20251201173012.18371-1-jacob.pan@linux.microsoft.com/

Thanks,

Jacob



Jacob Pan (8):
  iommufd: Add an ioctl IOMMU_IOAS_GET_PA to query PA from IOVA
  vfio: Allow null group for noiommu without containers
  vfio: Introduce and set noiommu flag on vfio_device
  vfio: Update noiommu device detection logic for cdev
  vfio: Enable cdev noiommu mode under iommufd
  vfio:selftest: Handle VFIO noiommu cdev
  selftests/vfio: Add iommufd noiommu mode selftest for cdev
  Doc: Update VFIO NOIOMMU mode

Jason Gunthorpe (3):
  iommufd: Support a HWPT without an iommu driver for noiommu
  iommufd: Move igroup allocation to a function
  iommufd: Allow binding to a noiommu device

 Documentation/driver-api/vfio.rst             |  44 +-
 drivers/iommu/iommufd/Makefile                |   1 +
 drivers/iommu/iommufd/device.c                | 161 +++--
 drivers/iommu/iommufd/hw_pagetable.c          |  11 +-
 drivers/iommu/iommufd/hwpt_noiommu.c          |  91 +++
 drivers/iommu/iommufd/io_pagetable.c          |  60 ++
 drivers/iommu/iommufd/ioas.c                  |  28 +
 drivers/iommu/iommufd/iommufd_private.h       |   5 +
 drivers/iommu/iommufd/main.c                  |   3 +
 drivers/vfio/Kconfig                          |   7 +-
 drivers/vfio/group.c                          |  35 +-
 drivers/vfio/iommufd.c                        |   7 -
 drivers/vfio/vfio.h                           |  34 +-
 drivers/vfio/vfio_main.c                      |  22 +-
 include/linux/vfio.h                          |  10 +
 include/uapi/linux/iommufd.h                  |  25 +
 tools/testing/selftests/vfio/Makefile         |   1 +
 .../selftests/vfio/lib/vfio_pci_device.c      |  25 +-
 .../vfio/vfio_iommufd_noiommu_test.c          | 549 ++++++++++++++++++
 19 files changed, 1027 insertions(+), 92 deletions(-)
 create mode 100644 drivers/iommu/iommufd/hwpt_noiommu.c
 create mode 100644 tools/testing/selftests/vfio/vfio_iommufd_noiommu_test.c

-- 
2.34.1