When powering on a VM after an ESXi update, and this error occurs:
The virtual machine requires hardware features that are unsupported or disabled on the target host: * Longmode: required for 64-bit guest OS support. On Intel systems, longmode requires VT-x to be enabled in the BIOS. On nested virtual ESX hosts, longmode support normally requires upgrading your VM to ESX 5.1 compatibility level (VM version 9) and enabling the "Virtualized Hardware Virtualization" flag on the outer VM. For incompatibilities other than NX/XD or longmode: if possible, use a cluster with Enhanced vMotion Compatibility (EVC) enabled; see KB article 1003212. CPUID details: incompatibility at level 0x80000001 register 'edx'. Host bits: 0000:1000:0001:0000:0000:1000:0000:0000 Required: xx1x:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx
it means that the new setting only available in the vSphere Web Client “Expose hardware-assisted virtualization to the guest operating system.” is not enabled.
ESXi 5.1 Virtualized HV is fully supported for virtual hardware version 9 VMs on hosts that support Intel VT-x and EPT or AMD-V and RVI. To enable virtualized HV, use the web client and navigate to the processor settings screen. Check the box next to "Expose hardware-assisted virtualization to the guest operating system." This setting is not available under the traditional C# client.