IOMMU

In computing, an input–output memory management unit (IOMMU) is a memory management unit (MMU) that connects a direct-memory-access–capable (DMA-capable) I/O bus to the main memory. Like a traditional MMU, which translates CPU-visible virtual addresses to physical addresses, the IOMMU maps device-visible virtual addresses (also called device addresses or I/O addresses in this context) to physical addresses. Some units also provide memory protection from faulty or malicious devices.

An example IOMMU is the graphics address remapping table (GART) used by AGP and PCI Express graphics cards on Intel Architecture and AMD computers.

On the x86 architecture, prior to splitting the functionality of northbridge and southbridge between the CPU and Platform Controller Hub (PCH), I/O virtualization was not performed by the CPU but instead by the chipset.

The advantages of having an IOMMU, compared to direct physical addressing of the memory, include:

For system architectures in which port I/O is a distinct address space from the memory address space, an IOMMU is not used when the CPU communicates with devices via I/O ports. In system architectures in which port I/O and memory are mapped into a suitable address space, an IOMMU can translate port I/O accesses.

The disadvantages of having an IOMMU, compared to direct physical addressing of the memory, include:

When an operating system is running inside a virtual machine, including systems that use paravirtualization, such as Xen, it does not usually know the host-physical addresses of memory that it accesses. This makes providing direct access to the computer hardware difficult, because if the guest OS tried to instruct the hardware to perform a direct memory access (DMA) using guest-physical addresses, it would likely corrupt the memory, as the hardware does not know about the mapping between the guest-physical and host-physical addresses for the given virtual machine. The corruption is avoided because the hypervisor or host OS intervenes in the I/O operation to apply the translations, incurring a delay in the I/O operation.

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