IOMMU in Relation To Virtualization
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; unfortunately, this delays the I/O operation.
An IOMMU can solve this problem by re-mapping the addresses accessed by the hardware according to the same (or a compatible) translation table that is used to map guest-physical address to host-physical addresses.
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