RHEL Engineering is moving the tracking of its product development work on RHEL 6 through RHEL 9 to Red Hat Jira (issues.redhat.com). If you're a Red Hat customer, please continue to file support cases via the Red Hat customer portal. If you're not, please head to the "RHEL project" in Red Hat Jira and file new tickets here. Individual Bugzilla bugs in the statuses "NEW", "ASSIGNED", and "POST" are being migrated throughout September 2023. Bugs of Red Hat partners with an assigned Engineering Partner Manager (EPM) are migrated in late September as per pre-agreed dates. Bugs against components "kernel", "kernel-rt", and "kpatch" are only migrated if still in "NEW" or "ASSIGNED". If you cannot log in to RH Jira, please consult article #7032570. That failing, please send an e-mail to the RH Jira admins at rh-issues@redhat.com to troubleshoot your issue as a user management inquiry. The email creates a ServiceNow ticket with Red Hat. Individual Bugzilla bugs that are migrated will be moved to status "CLOSED", resolution "MIGRATED", and set with "MigratedToJIRA" in "Keywords". The link to the successor Jira issue will be found under "Links", have a little "two-footprint" icon next to it, and direct you to the "RHEL project" in Red Hat Jira (issue links are of type "https://issues.redhat.com/browse/RHEL-XXXX", where "X" is a digit). This same link will be available in a blue banner at the top of the page informing you that that bug has been migrated.

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].

Published on December 17 version 2-24

Published on December 17 version 2-24