[PATCH v5 0/9] iommufd: Enable noiommu mode for cdev

[Jacob Pan <jacob.pan@linux.microsoft.com>]

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:

V5:
  - Split CONFIG_VFIO_NOIOMMU into CONFIG_VFIO_GROUP_NOIOMMU and
    CONFIG_VFIO_CDEV_NOIOMMU so cdev noiommu is independent of
    VFIO_GROUP (Alex)
  - Add CAP_SYS_RAWIO check for cdev open and bind under noiommu,
    security parity with group noiommu (Alex)
  - Add IS_ENABLED(CONFIG_IOMMUFD_NOIOMMU) guard in
    iommufd_device_is_noiommu() to prevent noiommu bind when feature
    is disabled
  - Add prep patch to tolerate NULL group for cdev noiommu devices
    when CONFIG_VFIO_GROUP_NOIOMMU is not set [7/9]
  - Rename IOCTL to IOMMUFD_CMD_IOAS_NOIOMMU_GET_PA to be more
    specific (Kevin)
  - Simplify iommufd_device_is_noiommu, use iommufd_bind_noiommu
    helper (Kevin, Yi)
  - Move IOMMU cap check under iommufd_bind_iommu() (Yi)
  - Fix next_iova exceeding iopt_area_last_iova in GET_PA (Alex)
  - Fix const hwpt, copyright date, typo in moved comment (Kevin)
  - Add Reviewed-by tags
  - Squash noiommu cdev selftest fix into selftest patch
  - Drop DSA selftest patch
  - Details in each patch changelog.

V4:
  - Fix various corner cases pointed out by (Sashiko)
    Details in each patch changelog.

V3:
  - Improve error handling [3/10] (Mostafa)
  - Simplify vfio_device_is_noiommu logic and merged in [6/10] (Mostafa)
  - Add comment to explain the design difference over the legacy noiommu
    VFIO code.[1/10]

V2:
  - Fix build dependency 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/

Future cleanup: consolidate all CONFIG_IOMMUFD_NOIOMMU code
(iopt_get_phys, iommufd_ioas_noiommu_get_pa, iommufd_noiommu_ops) into
hwpt_noiommu.c to eliminate #ifdef guards from ioas.c and io_pagetable.c.

Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
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2.43.0