34-20 Vol. 3C

SYSTEM MANAGEMENT MODE

34.15.2.1   Architectural State Before a VM Exit

System-management interrupts (SMIs) that cause SMM VM exits always do so directly. They do not save state to 
SMRAM as they do under the default treatment.

34.15.2.2   Updating the Current-VMCS and Executive-VMCS Pointers

SMM VM exits begin by performing the following steps:
1. The executive-VMCS pointer field in the SMM-transfer VMCS is loaded as follows:

— If the SMM VM exit commenced in VMX non-root operation, it receives the current-VMCS pointer.
— If the SMM VM exit commenced in VMX root operation, it receives the VMXON pointer.

2. The current-VMCS pointer is loaded with the value of the SMM-transfer VMCS pointer.
The last step ensures that the current VMCS is the SMM-transfer VMCS. VM-exit information is recorded in that 
VMCS, and VM-entry control fields in that VMCS are updated. State is saved into the guest-state area of that VMCS. 
The VM-exit controls and host-state area of that VMCS determine how the VM exit operates.

34.15.2.3   Recording VM-Exit Information

SMM VM exits differ from other VM exit with regard to the way they record VM-exit information. The differences 
follow.

•

Exit reason.
— Bits 15:0 of this field contain the basic exit reason. The field is loaded with the reason for the SMM VM exit: 

I/O SMI (an SMI arrived immediately after retirement of an I/O instruction), other SMI, or VMCALL. See 
Appendix C, “VMX Basic Exit Reasons”.

— SMM VM exits are the only VM exits that may occur in VMX root operation. Because the SMM-transfer 

monitor may need to know whether it was invoked from VMX root or VMX non-root operation, this 
information is stored in bit 29 of the exit-reason field (see Table 24-14 in Section 24.9.1). The bit is set by 
SMM VM exits from VMX root operation.

— If the SMM VM exit occurred in VMX non-root operation and an MTF VM exit was pending, bit 28 of the exit-

reason field is set; otherwise, it is cleared.

— Bits 27:16 and bits 31:30 are cleared.

•

Exit qualification. For an SMM VM exit due an SMI that arrives immediately after the retirement of an I/O 
instruction, the exit qualification contains information about the I/O instruction that retired immediately before 
the SMI. It has the format given in Table 34-9.

Table 34-9.  Exit Qualification for SMIs That Arrive Immediately After the Retirement of an I/O Instruction

Bit Position(s)

Contents

2:0

Size of access:

0 =  1-byte

1 =  2-byte

3 =  4-byte

Other values not used.

3

Direction of the attempted access (0 = OUT, 1 = IN)

4

String instruction (0 = not string; 1 = string)

5

REP prefixed (0 = not REP; 1 = REP)

6

Operand encoding (0 = DX, 1 = immediate)