Devices that do not support memory addresses long enough to address the entire physical memory can still address the entire memory through the IOMMU, avoiding overheads associated with copying buffers to and from the peripheral's addressable memory space.

Thus, the use of vectored I/O ( scatter-gather lists) can sometimes be avoided.

Large regions of memory can be allocated without the need to be contiguous in physical memory – the IOMMU maps contiguous virtual addresses to the underlying fragmented physical addresses.

The advantages of having an IOMMU, compared to direct physical addressing of the memory (DMA), include: 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. 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. This applies to both SR-IOV and PCI passthrough.ĭefault virtualized Linux deployments usually use VirtIO which is pretty fast to begin with Getting closer to the HW does have limitations however, it makes your VMs less portable for deployments that require live migration for example. PCI passthrough would be useful for say a VM that runs an intense database that would benefit from being attached to a FiberChannel SAN. The only thing faster than SR-IOV is PCI passthrough though in that case only one VM can make use of that device, not even the host operating system can use it. Many VMs can talk to an SR-IOV device concurrently with very low overhead. The HW knows it's being virtualized and can delegate a HW slice of itself to the VM. SR-IOV is different, the peripheral itself must carry the support. In which case I don't think you can multiplex devices, it's a lot like PCI passthrough before all these fancy virtualization instructions existed :). IOMMU sounds like a generic name for Intel VT-d and AMD IOV. IOMMU is a system specific IO mapping mechanism and can be used with most devices. It would help to know exactly what Motherboard/CPU is advertising this feature.

Just having it available doesn't make things faster. The only way IOMMU will help is if you start assigning HW resources directly to the VM.

Enable iommu in bios