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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

> @@ -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.

~Andrew

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