Vtd

VT-d, or Intel Virtualization Technology for Directed I/O, is Intel's implementation of IOMMU technology designed to optimize and secure direct memory access for I/O devices in virtualized environments. It enables a hypervisor to assign a PCI Express device directly to a virtual machine while maintaining memory and DMA isolation from other guests and the host. The technology relies on an IOMMU (I/O Memory Management Unit) that remaps device DMA addresses to physical memory and enforces security policies, including interrupt remapping.

Key features include DMA remapping, interrupt remapping, and the ability to provide security checks to prevent DMA-based attacks. VT-d works in conjunction with other virtualization extensions (such as VT-x) and requires hardware support in the CPU and motherboard chipset, as well as BIOS/UEFI support and enabling VT-d in firmware and the host OS or hypervisor.

Use cases include PCIe device passthrough to guests, enabling high-performance virtualized I/O for NICs, GPUs, storage controllers, or other devices, and improved isolation because devices are not shared via emulation. This enables more direct and efficient I/O for virtual machines.

Limitations include that some devices are not fully assignable due to IOMMU grouping, and in some cases, ACS or other grouping solutions are needed to separate devices. Enabling VT-d can add configuration overhead, not all hypervisors support all features, and remapping can incur small performance penalties.

VT-d is supported by hypervisors such as KVM, Xen, and VMware ESXi for device assignment. Note that AMD has a separate technology known as AMD-Vi (IOMMU).