[PATCH V2 00/11] iommufd: Enable noiommu mode for cdev

Jacob Pan
2026-03-12T15:56:39Z

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

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2.34.1