Juniper vMX – Error: Intel IOMMU status Disabled

When starting a Juniper vMX router you may get a warning from the vMX.sh script that shows that the IOMMU status is disabled. It will look like this:

Intel IOMMU status................................[Disabled]
Warning! IOMMU disabled. Pl enable in /boot/grub/grub.cfg

This seems to occur if the host OS is CentOS – the shell script to detect the IOMMU does not validate this correctly for CentOS hosts.

To verify that IOMMU is indeed enabled use the virt-host-validate command:

  QEMU: Checking for device assignment IOMMU support                         : PASS
  QEMU: Checking if IOMMU is enabled by kernel                               : PASS

This warning can be ignored if the IOMMU status passes. If the IOMMU status does not pass, you will need to enable IOMMU. I have included the instructions to do this for CentOS and Ubuntu KVM hosts below.

Enable IOMMU – CentOS

  1. Edit the file /etc/default/grub.
  2. Look for the GRUB_CMDLINE_LINUX variable. In my case the line looked like this by default:
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=centos_netvirt1/root rd.lvm.lv=centos_netvirt1/swap rhgb quiet"
  1. Add the intel_iommu=on option to the end of that GRUB_CMDLINE_LINUX variable. In my case I also have huge pages enabled due to the amount of RAM for the vFP, so the modified line looks like this:
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=centos_netvirt1/root rd.lvm.lv=centos_netvirt1/swap rhgb quiet default_hugepagesz=1G hugepagesz=1G hugepages=64 processor.max_cstates=1 idle=poll pcie_aspm=off intel_iommu=on"
  1. Save the updated configuration file.
  2. Generate the new Grub configuration with grub2-mkconfig:
grub2-mkconfig -o /boot/grub2/grub.cfg
  1. Reboot the host to apply the changes.

Note: The pcie_aspm and processor.max_cstates settings are not required but they are recommended by Juniper for best performance. The intel_iommu setting is required for SR-IOV support.

Enable IOMMU – Ubuntu

  1. Edit the file /etc/default/grub.
  2. Look for the two GRUB_CMDLINE_LINUX lines that look like this by default:
GRUB_CMDLINE_LINUX_DEFAULT=""
GRUB_CMDLINE_LINUX=""
  1. Add the intel_iommu=on option to the end of the GRUB_CMDLINE_LINUX_DEFAULT variable. In my case I also have huge pages enabled due to the amount of RAM for the vFP, so the modified lines look like this:
GRUB_CMDLINE_LINUX_DEFAULT="processor.max_cstates=1 idle=poll pcie_aspm=off intel_iommu=on"
GRUB_CMDLINE_LINUX="default_hugepagesz=1G hugepagesz=1G hugepages=64"
  1. Save the updated configuration file.
  2. Generate the new grub configuration file:
update-grub
  1. Reboot the host to apply the changes.

Note: The pcie_aspm and processor.max_cstates settings are not required but they are recommended by Juniper for best performance. The intel_iommu setting is required for SR-IOV support.

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