VMPTRLD - Load VMCS pointer

vmptrld will load the vmcs pointer for the virtual-machine to be launched. The vmcs stands for Virtual Machine Control Structure. The vmcs is a region in memory which holds all the data for the virtual-machine to be launched. The instruction usage is similar to vmxon:

vmptrld [vmcs_ptr]
vmcs_ptr dq vmcs_region

vmcs_region:
rev_id dd 0

As with vmxon, the revision id of the vmcs_region should be updated with the revision-id supported by the processor (contained in msr 0x480) prior to executing vmptrld. As with vmxon, the vmcs_region must be located on a 4K boundary.


The only other thing worth mentioning is if you try to load the vmxon_ptr as an operand to vmptrld, then execution of vmptrld will fail. Meaning, a code sequence like the one shown below is guaranteed to fail vmptrld:
vmxon [vmxon_ptr]
jbe vmxon_failed
vmptrld [vmxon_ptr]
jbe vmptrld_failed

When the processor executes vmptrld, it realizes that vmptrld's pointer points to the same region as vmxon. This will cuase vmptrld to fail.

It may also be a good practice to execute vmclear before executing vmptrld to load the vmcs-pointer. So the hypervisor may want to do this:
vmclear [vmcs_ptr]
jbe vmclear_failed
vmptrld [vmcs_ptr]
jbe vmptrld_failed

At this point we have executed vmxon, entered VMX_ROOT mode, initialized the virtual-machine-vmcs with vmclear and loaded the virtual-machine-vmcs pointer into the processor by executing vmptrld. The next step is to initialize the vmcs with the virtual-machine's (hence forth referred to as guest) data and then launch the guest.