Short answer: When the ESXi installer stops with CPU_SUPPORT ERROR: The CPU in this host is not supported by ESXi, reboot the installer and press Shift+O at the boot prompt. Append allowLegacyCPU=true and press Enter. The error becomes a CPU_SUPPORT WARNING that you can accept. This works on 7.0 and 8.0, and lab users report it on 9.x. It is not supported by Broadcom, so use it only in a homelab.
The error: “The CPU in this host is not supported by ESXi”
I first hit this on an older homelab server with the ESXi 7.0.0 installer (VMware-VMvisor-Installer-7.0.0-15843807.x86_64.iso):
CPU_SUPPORT ERROR:
The CPU in this host is not supported by ESXi
7.0.0. Please refer to the VMware Compatibility Guide (VCG) for
the list of supported CPUs.
The installer offers only Back or Reboot. Newer installers add a pointer to KB 82794, which is now Broadcom KB 318697. For what that KB says and which CPUs it lists, see KB 82794 explained.
Why the installer blocks the CPU
Broadcom KB 318697 defines two stages. A deprecated CPU series gets a warning and the installation continues. A discontinued series is blocked with an error. The check is part of the installer precheck. On the 7.0.0 ISO it lives in \UPGRADE\PRECHECK.PY, and when I looked at that file (around line 1720) it also contained the allowLegacyCPU override. If you only see a warning, your CPU is deprecated, not discontinued, and you need no override.
How to use allowLegacyCPU=true (Shift+O)
- Boot the ESXi installer ISO or USB.
- When the installer boot screen shows Shift+O: Edit boot options, press Shift+O. The prompt is only shown for a few seconds. Shift+O is the documented way to edit installer boot options.
- Leave the existing text (for example
runweasel cdromBoot) in place and append a space plus the option. - Press Enter. The system scan now reports a warning instead of an error. Press Enter to continue.
allowLegacyCPU=true
CPU_SUPPORT WARNING:
The CPU in this host is not supported by ESXi
7.0.0. Please refer to the VMware Compatibility Guide (VCG) for
the list of supported CPUs.Using allowLegacyCPU=true in boot.cfg and kickstart
Shift+O applies only to the current boot. To avoid typing it, put it on the kernelopt= line of boot.cfg on your installer media or PXE files. TechDocs documents that kernelopt= appends its string to the kernel boot options. William Lam also noted (for 7.0) that the option is not persistent after installation. For an installed host that needs it on every boot, he suggests adding it to kernelopt in both /bootbank/boot.cfg and /altbootbank/boot.cfg.
For unattended installs, the warning prompt still waits for Enter, so you also need a kickstart flag. I cover that separately in Automating an ESXi 8.0 install with allowLegacyCPU=true (kickstart).
Which ESXi versions honour allowLegacyCPU=true
| Version | Behaviour | Basis |
|---|---|---|
| ESXi 7.0 | Error becomes a warning; install continues | My own test on 7.0.0 (build 15843807); William Lam |
| ESXi 8.0 GA to 8.0 U1 | Error becomes a warning | William Lam |
| ESXi 8.0 U2 and later | Still honoured, but the CPU must support XSAVE (Intel Sandy Bridge / AMD Bulldozer or newer). Older CPUs such as Westmere stop with VMB: 716: Unsupported CPU. | William Lam |
| ESX 9.0 / 9.1 | Lab users report the same error-to-warning behaviour | Community reports only; not documented by Broadcom |
allowLegacyCPU is not listed in Broadcom’s installation documentation or in KB 313264 (additional kernel boot options). It is an engineering override that has never been guaranteed and may disappear in any build.
What allowLegacyCPU=true does not give you
- No support. The host runs on a CPU that is not in the Broadcom Compatibility Guide for that release.
- No guarantee for updates. Any patch or update can add a new hard requirement, as 8.0 U2 did with XSAVE. Keep the previous boot bank so you can roll back.
- Not the fix for a warning. For a deprecated CPU, patching with
esxcli software profile updateonly needs--no-hardware-warning(Broadcom KB 391610). That host is still supported. - Not a production path. For production hosts with discontinued CPUs, KB 428874 describes RPQ/TQ requests for “VCF Software Only” support through your account team. Broadcom has also published a supported override procedure for Intel Skylake-SP on VCF 9.x. See Intel Skylake and ESX 9 support.
Disclaimer: this is not officially supported by VMware by Broadcom. You run it at your own risk.
Sources
- Broadcom KB 318697: CPU Support Deprecation and Discontinuation In VCF Releases (legacy KB 82794)
- Broadcom KB 428874: Deprecated CPU Systems / Servers in ESX 9.0 and implications for support
- Broadcom KB 391610: ESXi patch/upgrade failed with the CPU support warning
- Broadcom TechDocs: Common Task Steps to Enter Boot Options (Shift+O)
- Broadcom TechDocs: About the boot.cfg File (ESX 9.0)
Further reading (William Lam, who first published the tip): Allow unsupported CPUs when upgrading to ESXi 7.0, Automating ESXi 8.0 install using allowLegacyCPU=true, ESXi 8.0 Update 2 requires XSAVE even with allowLegacyCPU=true.
