IOMMU - Advantages

Advantages

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

• Large regions of memory can be allocated without the need to be contiguous in physical memory — the IOMMU will take care of mapping contiguous virtual addresses to the underlying fragmented physical addresses. Thus, the use of vectored I/O (scatter-gather lists) can sometimes be avoided.
• For devices that do not support memory addresses long enough to address the entire physical memory, the device can still address the entire memory through the IOMMU. This avoids overhead associated with copying buffers to and from the memory space the peripheral can address.
  • For example, as of 2010 x86 computers can use more than 4 GiB of memory, enabled by the PAE feature in an x86 processor. Still, an ordinary 32-bit PCI device simply cannot address the memory above the 4 GiB boundary, and thus it cannot perform DMA to it. Without an IOMMU, the operating system would have to implement time-consuming bounce buffers (FreeBSD/Linux) also known as double buffers (Windows nomenclature).
• Memory protection from malicious or misbehaving devices: a device cannot read or write to memory that hasn't been explicitly allocated (mapped) for it. The memory protection is based on the fact that OS running on the CPU (see figure) exclusively controls both the MMU and the IOMMU. The devices are physically unable to circumvent or corrupt configured memory management tables.
  • With virtualization, guest operating systems can use hardware that is not specifically made for virtualization. Higher performance hardware such as graphics cards use DMA to access memory directly; in a virtual environment all the memory addresses are remapped by the virtual machine software, which causes DMA devices to fail. The IOMMU handles this remapping, allowing for the native device drivers to be used in a guest operating system.
• In some architectures IOMMU also performs hardware interrupt remapping, in a manner similar to standard memory address remapping.
• Peripheral memory paging can be supported by an IOMMU. A peripheral using the PCI-SIG PCIe Address Translation Services (ATS) Page Request Interface (PRI) extension can detect and signal the need for memory manager services.

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.