Vol. 3C 31-3

VIRTUAL-MACHINE MONITOR PROGRAMMING CONSIDERATIONS

Processors perform various checks while executing any VMX instruction. They follow well-defined error handling on 
failures. VMX instruction execution failures detected before loading of a guest state are handled by the processor 
as follows: 

•

If the working-VMCS pointer is not valid, the instruction fails by setting RFLAGS.CF to 1.

•

If the working-VMCS pointer is valid, RFLAGS.ZF is set to 1 and the proper error-code is saved in the VM-
instruction error field of the working-VMCS.

Software is required to check RFLAGS.CF and RFLAGS.ZF to determine the success or failure of VMX instruction 
executions.
The following items provide details regarding use of the VM-entry instructions (VMLAUNCH and VMRESUME):

•

If the working-VMCS pointer is valid, the state of the working VMCS may cause the VM-entry instruction to fail. 
RFLAGS.ZF is set to 1 and one of the following values is saved in the VM-instruction error field:
— 4: VMLAUNCH with non-clear VMCS.

If this error occurs, software can avoid the error by executing VMRESUME.

— 5: VMRESUME with non-launched VMCS.

If this error occurs, software can avoid the error by executing VMLAUNCH.

Figure 31-1.  VMX Transitions and States of VMCS in a Logical Processor

(a) VMX Operation and VMX Transitions

(b) State of VMCS and VMX Operation

Processor 

Operation

VMXON

VM Entry

VM Entry

VM Entry

VM Entry

VM Exit

VM Exit

VM Exit

VM Exit

VMXOFF

Outside 

VMX 

Operation

VMX Root

Operation

VMX 

Non-Root 

Operation

Legend:

Legend:

Inactive 

VMCS

Current VMCS

(working)

Active VMCS 

(not current)

Current VMCS

(controlling)

VMCS B

VMCS A

VMLAUNCH

VMRESUME

VMPTRLD B

VMCLEAR B

VM Exit

VM Exit

VMPTRLD A

VMPTRLD A

VMCLEAR A

VM Exit

VM Exit

VMLAUNCH

VMRESUME