Vol. 3C 27-21

VM EXITS

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If the processor supports either the 1-setting of the “load IA32_BNDCFGS” VM-entry control or that of the 
“clear IA32_BNDCFGS” VM-exit control, the contents of the IA32_BNDCFGS MSR are saved into the corre-
sponding field.

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The value of the SMBASE field is undefined after all VM exits except SMM VM exits. See Section 34.15.2.

27.3.2 

Saving Segment Registers and Descriptor-Table Registers

For each segment register (CS, SS, DS, ES, FS, GS, LDTR, or TR), the values saved for the base-address, segment-
limit, and access rights are based on whether the register was unusable (see Section 24.4.1) before the VM exit:

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If the register was unusable, the values saved into the following fields are undefined: (1) base address; 
(2) segment limit; and (3) bits 7:0 and bits 15:12 in the access-rights field. The following exceptions apply:
— CS.

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The base-address and segment-limit fields are saved.

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The L, D, and G bits are saved in the access-rights field.

— SS.

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DPL is saved in the access-rights field.

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On processors that support Intel 64 architecture, bits 63:32 of the value saved for the base address are 
always zero.

— DS and ES. On processors that support Intel 64 architecture, bits 63:32 of the values saved for the base 

addresses are always zero.

— FS and GS. The base-address field is saved.
— LDTR. The value saved for the base address is always canonical.

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If the register was not unusable, the values saved into the following fields are those which were in the register 
before the VM exit: (1) base address; (2) segment limit; and (3) bits 7:0 and bits 15:12 in access rights.

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Bits 31:17 and 11:8 in the access-rights field are always cleared. Bit 16 is set to 1 if and only if the segment is 
unusable.

The contents of the GDTR and IDTR registers are saved into the corresponding base-address and limit fields.

27.3.3 

Saving RIP, RSP, and RFLAGS

The contents of the RIP, RSP, and RFLAGS registers are saved as follows:

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The value saved in the RIP field is determined by the nature and cause of the VM exit:
— If the VM exit occurred in enclave mode, the value saved is the AEP of interrupted enclave thread (the 

remaining items do not apply).

— If the VM exit occurs due to by an attempt to execute an instruction that causes VM exits unconditionally or 

that has been configured to cause a VM exit via the VM-execution controls, the value saved references that 
instruction.

— If the VM exit is caused by an occurrence of an INIT signal, a start-up IPI (SIPI), or system-management 

interrupt (SMI), the value saved is that which was in RIP before the event occurred.

— If the VM exit occurs due to the 1-setting of either the “interrupt-window exiting” VM-execution control or 

the “NMI-window exiting” VM-execution control, the value saved is that which would be in the register had 
the VM exit not occurred.

— If the VM exit is due to an external interrupt, non-maskable interrupt (NMI), or hardware exception (as 

defined in Section 27.2.2), the value saved is the return pointer that would have been saved (either on the