Vol. 3C 25-1

CHAPTER 25

VMX NON-ROOT OPERATION

In a virtualized environment using VMX, the guest software stack typically runs on a logical processor in VMX non-
root operation. This mode of operation is similar to that of ordinary processor operation outside of the virtualized 
environment. This chapter describes the differences between VMX non-root operation and ordinary processor oper-
ation with special attention to causes of VM exits (which bring a logical processor from VMX non-root operation to 
root operation). The differences between VMX non-root operation and ordinary processor operation are described 
in the following sections:

•

Section 25.1, “Instructions That Cause VM Exits”

•

Section 25.2, “Other Causes of VM Exits”

•

Section 25.3, “Changes to Instruction Behavior in VMX Non-Root Operation”

•

Section 25.4, “Other Changes in VMX Non-Root Operation” 

•

Section 25.5, “Features Specific to VMX Non-Root Operation”

•

Section 25.6, “Unrestricted Guests”

Chapter 26, “VM Entries,” describes the data control structures that govern VMX non-root operation. Chapter 26, 
“VM Entries,” describes the operation of VM entries by which the processor transitions from VMX root operation to 
VMX non-root operation. Chapter 25, “VMX Non-Root Operation,” describes the operation of VM exits by which the 
processor transitions from VMX non-root operation to VMX root operation.
Chapter 28, “VMX Support for Address Translation,” describes two features that support address translation in VMX 
non-root operation. Chapter 29, “APIC Virtualization and Virtual Interrupts,” describes features that support virtu-
alization of interrupts and the Advanced Programmable Interrupt Controller (APIC) in VMX non-root operation.

25.1 

INSTRUCTIONS THAT CAUSE VM EXITS

Certain instructions may cause VM exits if executed in VMX non-root operation. Unless otherwise specified, such 
VM exits are “fault-like,” meaning that the instruction causing the VM exit does not execute and no processor state 
is updated by the instruction. Section 27.1 details architectural state in the context of a VM exit.
Section 25.1.1 defines the prioritization between faults and VM exits for instructions subject to both. Section 
25.1.2 identifies instructions that cause VM exits whenever they are executed in VMX non-root operation (and thus 
can never be executed in VMX non-root operation). Section 25.1.3 identifies instructions that cause VM exits 
depending on the settings of certain VM-execution control fields (see Section 24.6).

25.1.1 

Relative Priority of Faults and VM Exits

The following principles describe the ordering between existing faults and VM exits:

•

Certain exceptions have priority over VM exits. These include invalid-opcode exceptions, faults based on 
privilege level,

1

 and general-protection exceptions that are based on checking I/O permission bits in the task-

state segment (TSS). For example, execution of RDMSR with CPL = 3 generates a general-protection exception 
and not a VM exit.

2

•

Faults incurred while fetching instruction operands have priority over VM exits that are conditioned based on 
the contents of those operands (see LMSW in Section 25.1.3).

•

VM exits caused by execution of the INS and OUTS instructions (resulting either because the “unconditional I/O 
exiting” VM-execution control is 1 or because the “use I/O bitmaps control is 1) have priority over the following 
faults: 

1. These include faults generated by attempts to execute, in virtual-8086 mode, privileged instructions that are not recognized in that 

mode.

2. MOV DR is an exception to this rule; see Section 25.1.3.