How to Install and Run VMware ESXi on Unsupported CPU

You might encounter the Unsupported CPU error when installing or upgrading VMware ESXi hosts to 7.x or 8.x on hosts with older processor models.

VMB: 611: Unsupported CPU: Intel family 0x06, model 0x1a, stepping 0x5 Intel(R) Xeon(R) CPU E5520 @ 2.27GHz See http://www.vmware.com/resources/compatibility

The error looks like this on a clean ESXi installation:

Error: CPU_SUPPORT_ERROR: The CPU in this host is not supported by ESXI 6.7.0. Please refer ti the VMware Compatibility Guide (VCG) for the list of supported CPUs.

Or:
CPU_SUPPORT OVERRIDEWARNING: The CPUs in this host are not supported by ESXi 8.0.0. You can override and force install, but it is not officially supported nor recommended.

This issue occurs when installing VMware ESXi on servers with CPU models that VMware considers obsolete.

Refer to the VMware Compatibility Guide for a list of supported hardware for your version of ESXi (https://www.vmware.com/resources/compatibility/search.php). Select your processor model or family in the search bar and select the version of ESXi you want to install. If your CPU is not listed, it is not officially supported.

For a list of supported and unsupported processors, see CPU Support Deprecation and Discontinuation in vSphere Releases (https://kb.vmware.com/s/article/82794).

If your processor is on the Deprecated list, it means that it will be discontinued in the next ESXi release. When installing VMware ESXi 8.x on a server with such a CPU, a warning is displayed:
The CPUs in this host may not be supported in future ESXi releases. Please plan accordingly
The ESXi installer blocks installation on such hardware if your processor is on the Discontinued list.

For example:

ESXi 6.7 and later cannot be installed on 6th generation Intel processors (Intel Family 6) based on the Westmere microarchitecture (manufactured since 2010).
ESXi 8.0 Update 2– no longer supports Intel Sandy Bridge, Ivybridge, Haswell, Broadwell, and AMD Bulldozer processors (released after 2011). ESXi can only be installed on CPUs that support the XSAVE instruction set.

I ran into this issue on an HP ProLiant DL380 G6 server with an Intel Xeon E5520 processor.

Allow VMware ESXi to Run on an Unsupported CPU

To run ESXi on a legacy CPU model, you need to add the “allowLegacyCPU=true” parameter to the ESXi boot menu.

This workaround is not officially supported by VMware.

To do this, press Shift+O(the letter O, not the number zero, is used here) while booting ESXi and run the command:

<ENTER: Apply options and boot>

 cdromBoot runweasel allowLegacyCPU=true

However, this setting is only valid until the next reboot of the ESXi host. Therefore, you need to add a permanent rule to the boot.cfg file. This file is located at the root of one of the system partitions.

Connect to the ESXi host via SSH and locate the boot.cfg file:

# find / | grep boot.cfg

In my case, I found two files (/bootbank/boot.cfg and /altbootbank/boot.cfg), so I edited both. Add the allowLegacyCPU=true option to the kernelopt parameter. For example:

bootstate=0 title=Loading VMware ESXi timeout=5 prefix= kernel=b.b00 kernelopt=autoPartition=FALSE allowLegacyCPU=true modules=jumpstrt.gz --- useropts.gz --- features.gz --- k.b00 --- uc_intel.b00 --- uc_amd.b00 --- uc_hygon.b00 --- procfs.b00 --- vmx.v00 --- vim.v00 --- tpm.v00 --- sb.v00 --- s.v00 --- bnxtnet.v00 --- bnxtroce.v00 --- brcmfcoe.v00 --- brcmnvme.v00 --- elxiscsi.v00 --- elxnet.v00 --- i40en.v00 --- i40iwn.v00 --- iavmd.v00 --- igbn.v00 --- iser.v00 --- ixgben.v00 --- lpfc.v00 --- lpnic.v00 --- lsi_mr3.v00 --- lsi_msgp.v00 --- lsi_msgp.v01 --- lsi_msgp.v02 --- mtip32xx.v00 --- ne1000.v00 --- nenic.v00 --- nfnic.v00 --- nhpsa.v00 --- nmlx4_co.v00 --- nmlx4_en.v00 --- nmlx4_rd.v00 --- nmlx5_co.v00 --- nmlx5_rd.v00 --- ntg3.v00 --- nvme_pci.v00 --- nvmerdma.v00 --- nvmxnet3.v00 --- nvmxnet3.v01 --- pvscsi.v00 --- qcnic.v00 --- qedentv.v00 --- qedrntv.v00 --- qfle3.v00 --- qfle3f.v00 --- qfle3i.v00 --- qflge.v00 --- rste.v00 --- sfvmk.v00 --- smartpqi.v00 --- vmkata.v00 --- vmkfcoe.v00 --- vmkusb.v00 --- vmw_ahci.v00 --- crx.v00 --- elx_esx_.v00 --- btldr.v00 --- esx_dvfi.v00 --- esx_ui.v00 --- esxupdt.v00 --- tpmesxup.v00 --- weaselin.v00 --- loadesx.v00 --- lsuv2_hp.v00 --- lsuv2_in.v00 --- lsuv2_ls.v00 --- lsuv2_nv.v00 --- lsuv2_oe.v00 --- lsuv2_oe.v01 --- lsuv2_oe.v02 --- lsuv2_sm.v00 --- native_m.v00 --- qlnative.v00 --- vdfs.v00 --- vmware_e.v00 --- vsan.v00 --- vsanheal.v00 --- vsanmgmt.v00 --- xorg.v00 --- imgdb.tgz --- state.tgz build=6.7.0-1.25.18828794 updated=1

Then add this option to the /altbootbank/boot.cfg file as well.

Note that the allowLegacyCPU option does not work during the upgrade to ESXi 8.0 Update 2.

VMB: 716: Unsupported CPU

In this case, the ESXi installer will abort the update. In this release, you cannot install ESXi on a device with a CPU on the End of Support or End of Life list.

Unable to Start VMware VM on Old CPUs

Another error might occur when you try to start a virtual machine on an ESXi host with an old CPU model:

Failed to power on virtual machine on xxx. This host doesn’t support virtualizing real mode. The intel VMX Unrestricted Guest feature is necessary to run this virtual machine on an Intel Processor.

To solve this problem, you need to add a special parameter to the virtual machine’s VMX configuration file:

Select VM -> Edit
Go to VM Options -> Advanced -> Edit Configuration
Click Add parameter and add the line monitor.allowLegacyCPU = true
You can then start the virtual machine.

“Fatal CPU Mismatch on Feature” Error During VMware ESXi Installation

Users may also experience a PSOD when installing ESXi 7.x and 8.x on servers with the latest 11th, 12th, and 13th generation Intel processors:

HW feature incompatibility detected; cannot start Fatal CPU mismatch on feature "Hyperthreads per core" Fatal CPU mismatch on feature "Cores per package" Fatal CPU mismatch on feature "Cores per die"

This issue relates to Intel’s new CPU core architecture. To support these processors, cpuUniformityHardCheckPanic must be enabled in ESXi 7.0 Update 2 or later.

Press Shift + O when the ESXi installer is launched;
Add a boot option cpuUniformityHardCheckPanic=FALSE
Proceed to ESXi Installation;
Open the ESXi shell (Alt+F1) and edit /vmfs/volumes/BOOTBANK1/boot.cfg without rebooting the host. Add the following parameter to the kernelopt option:

kernelopt=weaselInstalled autoPartition=FALSE cpuUniformityHardCheckPanic=FALSE

You can also add this parameter using ESXCLI:

# esxcli system settings kernel set -s  cpuUniformityHardCheckPanic -v FALSE

For Intel Raptor Lake processors (13th generation), you will also need to add an option:

# esxcli system settings kernel set -s ignoreMsrFaults -v TRUE