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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> @@ -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.

Jan

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