IOMMU

An IOMMU, or input-output memory management unit, is a hardware block that translates the addresses a device uses for direct memory access (DMA) and limits which memory the device can touch. Intel's version is VT-d, Virtualization Technology for Directed I/O. AMD's is AMD-Vi. Firmware lists the units in ACPI tables, DMAR on Intel and IVRS on AMD. With one active, a hypervisor can assign a PCIe device such as a GPU to a virtual machine, and the guest cannot write over host memory.

Linux sorts devices into IOMMU groups, the smallest set the hardware can isolate from everything else, and VFIO assigns a whole group at a time. Groups get wider behind a PCIe-to-PCI bridge or a bridge without ACS, Access Control Services. If a group holds several devices, each must be bound to a VFIO driver first. On Intel, interrupt addresses from 0xfee00000 to 0xfeefffff are not translated. Intel publishes VT-d as document D51397, revision 5.20 dated April 2026, and AMD publishes its IOMMU specification as document 48882.

Enable VT-d or AMD-Vi in the BIOS before building a passthrough host. Then read the group layout on the actual board. A GPU sharing a group with a USB or SATA controller moves with it.

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Last verified October 30, 2026.