Re: [Xen-devel] [PATCH] x86/emul: Reorder the user segments in x86_segment to match SReg3 encoding

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/10/16 10:16, Jan Beulich wrote:
>>>> On 14.10.16 at 18:13, <andrew.cooper3@xxxxxxxxxx> wrote:
>> @@ -2980,8 +2964,8 @@ x86_emulate(
>>          break;
>>  
>>      case 0x8c: /* mov Sreg,r/m */
>> -        seg = decode_segment(modrm_reg);
>> -        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
>> +        seg = modrm_reg;
> You need to mask with 7 here and ...
>
>> @@ -2992,8 +2976,8 @@ x86_emulate(
>>          break;
>>  
>>      case 0x8e: /* mov r/m,Sreg */
>> -        seg = decode_segment(modrm_reg);
>> -        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
>> +        seg = modrm_reg;
> ... here - I've just checked that at least Intel ignores REX.R.

Both points are covered by by the is_x86_user_segment() check which you
have cropped out of context.

One option, if you would prefer, is to reverse the check and assignment,
passing modrm_reg into is_x86_user_segment(), but I don't see any need
for other code changes.

>
>> @@ -5520,4 +5504,15 @@ x86_insn_length(const struct x86_emulate_state *state,
>>      return state->eip - ctxt->regs->eip;
>>  }
>>  
>> +static void __init __maybe_unused build_assertions(void)
>> +{
>> +    /* Check the values against SReg3 encoding in opcode/ModRM bytes. */
>> +    BUILD_BUG_ON(x86_seg_es != 0);
>> +    BUILD_BUG_ON(x86_seg_cs != 1);
>> +    BUILD_BUG_ON(x86_seg_ss != 2);
>> +    BUILD_BUG_ON(x86_seg_ds != 3);
>> +    BUILD_BUG_ON(x86_seg_fs != 4);
>> +    BUILD_BUG_ON(x86_seg_gs != 5);
>> +}
> If we really want this, I think it should be enabled for the test tool
> too.

I wonder whether I can get away with having it as a static inline in
x86_emulate.h

~Andrew

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