Bug 1201503

Document URL: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Virtualization_Deployment_and_Administration_Guide/chap-Guest_virtual_machine_device_configuration.html#sect-Guest_virtual_machine_device_configuration-PCI_devices

Section Number and Name: 20.1

Describe the issue: 

When preparing RHEL for PCI device assignment (for a KVM VM), IOMMU feature is required.  Currently, the document instructs to use "iommu-on".  This does work, but it can have significant performance side affects.  "iommu=on" enables IOMMU for all devices, even if they are not used for device assignment by KVM.  When using IOMMU for devices in the host, extra overhead is introduced for each DMA operation.  This can reduce performance of some IO adapters like 10Gb networking significantly.

Suggestions for improvement: 

The most appropriate option to use for device assignment is "iommu=pt".  The enables the IOMMU only for device assignment, which is exactly what is needed.

Additional information: 

If we want to document the use for "iommu=on" then it should be in another area, as the use case for that option is not actually for device assignment.  It is to provide protection to the host from a malfunctioning device driver [in the host].