Re: [PATCH 01/43] x86/decoder: Add new TEST instruction pattern

[Borislav Petkov]

On Fri, Nov 24, 2017 at 10:14:06AM +0100, Ingo Molnar wrote:
> From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> 
> The kbuild test robot reported this build warning:
> 
>   Warning: arch/x86/tools/test_get_len found difference at <jump_table>:ffffffff8103dd2c
> 
>   Warning: ffffffff8103dd82: f6 09 d8 testb $0xd8,(%rcx)
>   Warning: objdump says 3 bytes, but insn_get_length() says 2
>   Warning: decoded and checked 1569014 instructions with 1 warnings
> 
> This sequence seems to be a new instruction not in the opcode map in the Intel SDM.
> 
> The instruction sequence is "F6 09 d8", means Group3(F6), MOD(00)REG(001)RM(001), and 0xd8.

So that's TEST Eb,Ib with ModRM.reg == 1b which is documented in the AMD APM but
not in the Intel SDM.

Maybe H.J. has some insights on why.

CCed and leaving in the rest for reference.

> Intel SDM vol2 A.4 Table A-6 said the table index in the group is "Encoding of Bits 5,4,3 of
> the ModR/M Byte (bits 2,1,0 in parenthesis)"
> 
> In that table, opcodes listed by the index REG bits as:
> 
>   000         001       010 011  100        101        110         111
>  TEST Ib/Iz,(undefined),NOT,NEG,MUL AL/rAX,IMUL AL/rAX,DIV AL/rAX,IDIV AL/rAX
> 
> So, it seems TEST Ib is assigned to 001.
> 
> Add the new pattern.
> 
> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  arch/x86/lib/x86-opcode-map.txt | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/lib/x86-opcode-map.txt b/arch/x86/lib/x86-opcode-map.txt
> index 12e377184ee4..c4d55919fac1 100644
> --- a/arch/x86/lib/x86-opcode-map.txt
> +++ b/arch/x86/lib/x86-opcode-map.txt
> @@ -896,7 +896,7 @@ EndTable
>  
>  GrpTable: Grp3_1
>  0: TEST Eb,Ib
> -1:
> +1: TEST Eb,Ib
>  2: NOT Eb
>  3: NEG Eb
>  4: MUL AL,Eb
> -- 
> 2.14.1
> 

-- 
Regards/Gruss,
    Boris.

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