Re: [Qemu-discuss] How to check cpu running mode?

[krishnaLee]

it looks excellent code,although I'm not very familiar with ASM code,I will dig 
into it,
thank you,Jakob!


by krishna.








At 2018-08-17 22:00:40, "Jakob Bohm" <address@hidden> wrote:
>On 17/08/2018 15:32, Jakob Bohm wrote:
>> On 17/08/2018 12:27, krishnaLee wrote:
>>> Hi,
>>> as to my known,intel  x64 cpu can work at 64-bit mode or 
>>> Compatibility mode,
>>> so thery are two kind of  x64-mode,
>>> I find QEMU can run at each of them,
>>> I want  to write som C code to check it if it is running at 64-bit 
>>> mode or Compatibility mode, what's the key?
>>>
>> This mode is a bit in the segment descriptor referenced by the
>> current CS register, the following or similar code should be
>> able to check this(note that this code has to use only
>> instructions that have the same encoding in all CPU modes!):
>>
>>     ; UNTESTED HYPOTHETICAL CODE!!!
>>
>>     ; First, check that we are not running in real or VM8086
>>     ;    mode, because then the following will not work
>>     ;    (LAR will trigger an undefined opcode exception).
>>     ; Doing this is left as an exercise for the reader
>>
>>     ; The following is valid in 16, 32 and 64 bit protected mode
>>     ;    the instructions should have the same opcodes, they are
>>     ;    shown here using the 32 bit mode Intel syntax
>>     .386
>>     MOV EAX,CS
>>     LAR EAX,EAX
>>     JNZ SHORT modeerror
>>     SHR EAX,21
>>     JCSHORT mode64
>>     SHR EAX,1
>>     JC SHORT mode32
>> mode16j:
>>     .286
>>     ; Code here is 16 bit code and can use all 16 bit instructions
>>     JMP mode16    ; This can be a longer jmp now that we know the
>>                   ; instruction encoding
>> mode32j:
>>     .386
>>     ; Code here is 32 bit code and can use all 32 bit instructions
>>     JMP mode32    ; This can be a longer jmp now that we know the
>>                   ; instruction encoding
>> mode64j:
>>     .amd64   ; Note, not sure what directive tells MASM to generate 
>> 64bit opcodes
>>     ; Code here is 64 bit code and can use all 64 bit instructions
>>     JMP mode64    ; This can be a longer jmp now that we know the
>>                   ; instruction encoding
>> modeerror:
>>     ; This really shouldn't happen!
>>     ; One possible way would be if something changed the CS descriptor
>>     ;    in the descriptor table without reloading CS
>>     ; Code here is still restricted to mode independent instructions!
>>
>> ; Alternatively, a code sequence could be constructed that has different
>> ;    effects depending on CPU mode but doesn't directly query the CS
>> ;    descriptor table entry, avoiding the modeerror case above.
>>
>>     ; UNTESTED HYPOTHETICAL CODE!!!
>>
>> ; Perhaps
>>     .386
>>     XOR EAX,EAX   ; XOR AX,AX if 16 bit mode
>>     NOT EAX       ; 16 bit: AX is now 0FFFFh
>>                   ; 32 bit: EAX is now 0FFFFFFFFh
>>                   ; 64 bit: RAX is now 0FFFFFFFFFFFFFFFFh (UNTESTED)
>>     SHR EAX,16
>>     JNC SHORT mode16j
>> SHR EAX,16
>>     JNC SHORT mode32j
>> mode64j:
>>     ; as above
>> mode32j:
>>     ; as above
>> mode16j:
>>     .286
>>     SMSW AX
>>     SHR AX,1
>>     JNC SHORT mode16real
>>     ; Add code here to test if this is VM8086 mode or 16 bit
>>     ;    protected mode
>>     ; First set up a stack, 4 byte aligned
>>     ; CODE MISSING
>>     ; Next do the classic tricks to detect if this is at least
>>     ;    a 386, this involves using 16 bit PUSHF and POPF
>>     ; CODE MISSING
>>     ;    The missing code should jump to mode16rj if < 386
>>     ; Next use code to determine if in VM8086 mode, note that
>>     ;    PUSHFD hides the desired EFLAGS bit from us, so another
>>     ;    trick is needed
>>     ; CODE MISSING
>>    ;    The missing code should jump to mode16rj if real mode
>> mode16vj:
>>     .286
>>     ; Code here is 16 bit code and can use all 16 bit instructions
>>     ; VM8086 mode
>>     JMP mode16v   ; This can be a longer jmp now that we know the
>>                   ; instruction encoding
>> mode16rj:
>>     .286
>>     ; Code here is 16 bit code and can use all 16 bit instructions
>>     ; Real mode, full CPU access!
>>     JMP mode16v   ; This can be a longer jmp now that we know the
>>                   ; instruction encoding
>Corrections:
>
>In the first example, the first shift should be
>     SHR EAX,22
>
>In the second example, the first shift should be
>     SHR EAX,17
>
>And the entire 16 bit case needs to be regrouped:
>
>mode16rj:
>     .286
>     ; Code here is 16 bit code and can use all 16 bit instructions
>     ; Real mode, full CPU access!
>     ; May or may not be the unofficial large-segment real mode
>     JMP mode16r   ; This can be a longer jmp now that we know the
>                   ; instruction encoding
>mode16j:
>     .286
>     ; 16 bit code, maybe protected, real, VM8086 or even
>     ;    the unofficial large-segment real mode
>
>     ; First set up a stack, 4 byte aligned
>     ; CODE MISSING OR ASSUME STACK ALREADY VALID
>     ; Next do the classic tricks to detect if this is at least
>     ;    a 286, this involves using 16 bit PUSHF and POPF
>     ; CODE MISSING
>     ;    The missing code should jump to mode16rj if < 286
>     SMSW AX
>     SHR AX,1
>     JNC SHORT mode16rj
>     ; So now it is protected mode or VM8086 mode
>     ; First check if at least a 386 (we know it's at least 286)
>     ; CODE MISSING
>     ;    The missing code should jump to mode16pj if a 286
>     ; Add code here to test if this is VM8086 mode or 16 bit
>     ;    protected mode
>     ; Next use code to determine if in VM8086 mode, note that
>     ;    PUSHFD hides the desired EFLAGS bit from us, so another
>     ;    trick is needed
>     ; CODE MISSING
>     ;    The missing code should jump to mode16vj if VM8086 mode
>mode16pj:
>     .286
>     ; Code here is 16 bit code and can use all 16 bit instructions
>     ; protected mode 16 bit
>     JMP mode16p   ; This can be a longer jmp now that we know the
>                   ; instruction encoding
>mode16vj:
>     .286
>     ; Code here is 16 bit code and can use all 16 bit instructions
>     ; VM8086 mode
>     JMP mode16v   ; This can be a longer jmp now that we know the
>                   ; instruction encoding
>
>Enjoy
>
>Jakob
>-- 
>Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
>Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
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>