Re: [Xen-devel] [PATCH 5/5] x86/emulate: add support for {, v}movd {, x}mm, r/m32 and {, v}movq {, x}mm, r/m64

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/09/16 14:46, Jan Beulich wrote:
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -223,7 +223,7 @@ static const opcode_desc_t twobyte_table
>      /* 0x70 - 0x7F */
>      SrcImmByte|ModRM, SrcImmByte|ModRM, SrcImmByte|ModRM, SrcImmByte|ModRM,
>      ModRM, ModRM, ModRM, ImplicitOps,
> -    ModRM, ModRM, 0, 0, ModRM, ModRM, ModRM, ImplicitOps|ModRM,
> +    ModRM, ModRM, 0, 0, ModRM, ModRM, ImplicitOps|ModRM, ImplicitOps|ModRM,
>      /* 0x80 - 0x87 */
>      DstImplicit|SrcImm, DstImplicit|SrcImm,
>      DstImplicit|SrcImm, DstImplicit|SrcImm,
> @@ -2291,6 +2291,10 @@ x86_decode(
>          return X86EMUL_UNHANDLEABLE;
>      }
>  
> +    if ( op_bytes == 2 &&
> +         (ctxt->opcode & X86EMUL_OPC_PFX_MASK) == X86EMUL_OPC_66(0, 0) )
> +        op_bytes = 4;

What is this change for?  I presume it is to undo the effect of the
operand size override prefix when we have decided that the prefix
actually had an alternate meaning?

If so, can we have a comment to this effect?

Everything else looks ok.

~Andrew

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