No blocking of A20M during VMX operation

[lgj1107]

According to the manual 24.8 RESTRICTIONS ON VMX OPERATION:

VMXON fails if a logical processor is in A20M mode (see “VMXON—Enter VMX Operation” in Chapter 31). Once
the processor is in VMX operation, A20M interrupts are blocked. Thus, it is impossible to be in A20M mode in
VMX operation.

Bochs internal debugger, type 'help' for help or 'c' to continue
Next at t=0
(0) [0x0000fffffff0] f000:fff0 (unk. ctxt): jmpf 0xf000:e05b ; ea5be000f0
bochs:1 c
(0) Magic breakpoint
Next at t=337329509
(0) [0x000000100002] 0000:0002 (unk. ctxt): xor al, al ; 30c0
bochs:2 page 0x100000
EPT PML4: 0x0000000070801007 X W R
EPT PDPE: 0x0000000070802007 X W R
EPT PDE: 0x0000000070824007 X W R
EPT PTE: 0x0000000000100037 X W R ignore_pat WB
linear page 0x0000000000100000 maps to physical page 0x000000100000
bochs:3 s
Next at t=337329510
(0) [0x000000100004] 0000:0004 (unk. ctxt): out 0x92, al ; e692
bochs:4 s
Next at t=337329511
(0) [0x000000000006] 0000:0006 (unk. ctxt): add al, dh ; 00f0 <------wrap
bochs:5 page 0x100000
EPT PML4: 0x0000000070801007 X W R
EPT PDPE: 0x0000000070802007 X W R
EPT PDE: 0x0000000070824007 X W R
EPT PTE: 0x0000000000100037 X W R ignore_pat WB
linear page 0x0000000000100000 maps to physical page 0x000000000000
bochs:6

test1.zip

[stlintel]

Do you expect VMXON to fail instead ?

[lgj1107]

My understanding is that after executing vmxon (entering VMX operation), A20M is blocked, so xor al,al out 0x92,al will not affect the A20 line.