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What: /sys/kernel/iommu_groups/
Date: May 2012
KernelVersion: v3.5
Contact: Alex Williamson <>
Description: /sys/kernel/iommu_groups/ contains a number of sub-
directories, each representing an IOMMU group. The
name of the sub-directory matches the iommu_group_id()
for the group, which is an integer value. Within each
subdirectory is another directory named "devices" with
links to the sysfs devices contained in this group.
The group directory also optionally contains a "name"
file if the IOMMU driver has chosen to register a more
common name for the group.
What: /sys/kernel/iommu_groups/reserved_regions
Date: January 2017
KernelVersion: v4.11
Contact: Eric Auger <>
Description: /sys/kernel/iommu_groups/reserved_regions list IOVA
regions that are reserved. Not necessarily all
reserved regions are listed. This is typically used to
output direct-mapped, MSI, non mappable regions. Each
region is described on a single line: the 1st field is
the base IOVA, the second is the end IOVA and the third
field describes the type of the region.
Since kernel 5.3, in case an RMRR is used only by graphics or
USB devices it is now exposed as "direct-relaxable" instead
of "direct". In device assignment use case, for instance,
those RMRR are considered to be relaxable and safe.
What: /sys/kernel/iommu_groups/<grp_id>/type
Date: November 2020
KernelVersion: v5.11
Contact: Sai Praneeth Prakhya <>
Description: /sys/kernel/iommu_groups/<grp_id>/type shows the type of default
domain in use by iommu for this group. See include/linux/iommu.h
for possible read values. A privileged user could request kernel to
change the group type by writing to this file. Valid write values:
======== ======================================================
DMA All the DMA transactions from the device in this group
are translated by the iommu.
DMA-FQ As above, but using batched invalidation to lazily
remove translations after use. This may offer reduced
overhead at the cost of reduced memory protection.
identity All the DMA transactions from the device in this group
are not translated by the iommu. Maximum performance
but zero protection.
auto Change to the type the device was booted with.
======== ======================================================
The default domain type of a group may be modified only when
- The group has only one device.
- The device in the group is not bound to any device driver.
So, the users must unbind the appropriate driver before
changing the default domain type.
Unbinding a device driver will take away the driver's control
over the device and if done on devices that host root file
system could lead to catastrophic effects (the users might
need to reboot the machine to get it to normal state). So, it's
expected that the users understand what they're doing.