Re: [Xen-devel] [PATCH] x86emul: VEX.B is ignored in compatibility mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.01.17 at 18:29, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 12/01/17 16:37, Jan Beulich wrote:
>> While VEX.R and VEX.X are guaranteed to be 1 in compatibility mode,
>> VEX.B can be encoded as zero, but would be ignored by the processor.
> 
> Really?  That is unfortunate.
> 
> It would have been far more helpful for this to raise #UD, like the
> other prohibited VEX encodings.

It's spelled out that way in the doc, and to be sure I've just again
verified this to be the case in practice.

>> @@ -2235,7 +2241,7 @@ x86_decode(
>>                          break;
>>                      }
>>                  }
>> -                if ( mode_64bit() && !vex.r )
>> +                if ( !vex.r )
>>                      rex_prefix |= REX_R;
>>  
>>                  ext = vex.opcx;
>>
> 
> What is the purpose of this change? I doesn't appear to be related to
> the rest of the patch.

It is related - see the first half of the first sentence of the description
(still visible above).

Jan


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