When This May Be Required

1. Host fails to boot after an update due to a microcode-related error
2. Troubleshooting to determine whether a boot issue is microcode-related
3. At engineering or support’s direction to isolate a problem

⚠️ Warning: Disabling microcode loading may leave the host without the latest CPU security mitigations. This should be treated as a temporary diagnostic measure.

Note: This change applies to the current boot only and is lost on the next reboot.

XenServer 8.4

1. Restart the host
2. At the GRUB menu, highlight the XenServer boot entry and press e to edit
3. Locate the line beginning with multiboot2 - this contains the Xen hypervisor parameters
4. Find ucode=scan on that line and :

Delete ucode=scan entirely

5. Press Ctrl+X or F10 to boot with the modified parameters

XenServer 9

1. Restart the host
2. At the GRUB menu, highlight the XenServer boot entry and press e to edit
3. Locate the line beginning with xen_hypervisor - this contains the Xen hypervisor parameters
4. Append ucode=no-scan to the end of the line

5. Press Ctrl+X or F10 to boot with the modified parameters

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Overview

By default XenServer will scan for included microcode updates and apply them during boot. In some scenarios such as troubleshooting boot failures related to microcode, or temporary diagnostics you may need to disable this behaviour.

This article describes how to disable microcode loading temporarily (for a single boot).