Appendix G. Configuring a Host for PCI Passthrough

Enabling PCI passthrough allows a virtual machine to use a host device as if the device were directly attached to the virtual machine. To enable the PCI passthrough function, you need to enable virtualization extensions and the IOMMU function. The following procedure requires you to reboot the host. If the host is attached to the Manager already, ensure you place the host into maintenance mode before running the following procedure.

Prerequisites:

Configuring a Host for PCI Passthrough

  1. Enable the virtualization extension and IOMMU extension in the BIOS. See Enabling Intel VT-x and AMD-V virtualization hardware extensions in BIOS in the Red Hat Enterprise Linux Virtualization and Administration Guide for more information.
  2. Enable the IOMMU flag in the kernel by selecting the Hostdev Passthrough & SR-IOV check box when adding the host to the Manager or by editing the grub configuration file manually.

  3. For GPU passthrough, you need to run additional configuration steps on both the host and the guest system. See Preparing Host and Guest Systems for GPU Passthrough in the Administration Guide for more information.

Enabling IOMMU Manually

  1. Enable IOMMU by editing the grub configuration file.

    Note

    If you are using IBM POWER8 hardware, skip this step as IOMMU is enabled by default.

    • For Intel, boot the machine, and append intel_iommu=on to the end of the GRUB_CMDLINE_LINUX line in the grub configuration file.

      # vi /etc/default/grub
      ...
      GRUB_CMDLINE_LINUX="nofb splash=quiet console=tty0 ... intel_iommu=on
      ...
    • For AMD, boot the machine, and append amd_iommu=on to the end of the GRUB_CMDLINE_LINUX line in the grub configuration file.

      # vi /etc/default/grub
      ...
      GRUB_CMDLINE_LINUX="nofb splash=quiet console=tty0 ... amd_iommu=on
      ...
      Note

      If intel_iommu=on or amd_iommu=on works, you can try adding iommu=pt or amd_iommu=pt. The pt option only enables IOMMU for devices used in passthrough and provides better host performance. However, the option might not be supported on all hardware. Revert to previous option if the pt option doesn’t work for your host.

      If the passthrough fails because the hardware does not support interrupt remapping, you can consider enabling the allow_unsafe_interrupts option if the virtual machines are trusted. The allow_unsafe_interrupts is not enabled by default because enabling it potentially exposes the host to MSI attacks from virtual machines. To enable the option:

      # vi /etc/modprobe.d
      options vfio_iommu_type1 allow_unsafe_interrupts=1
  2. Refresh the grub.cfg file and reboot the host for these changes to take effect:

    # grub2-mkconfig -o /boot/grub2/grub.cfg
    # reboot

For enabling SR-IOV and assigning dedicated virtual NICs to virtual machines, see https://access.redhat.com/articles/2335291 for more information.