Re: [Xen-devel] [PATCH v2 04/16] x86emul: complete decoding of two-byte instructions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/09/16 09:08, Jan Beulich wrote:
> @@ -1651,6 +1668,34 @@ x86_decode_onebyte(
>  }
>  
>  static int
> +x86_decode_twobyte(
> +    struct x86_emulate_state *state,
> +    struct x86_emulate_ctxt *ctxt,
> +    const struct x86_emulate_ops *ops)
> +{
> +    int rc = X86EMUL_OKAY;
> +
> +    switch ( state->opcode )
> +    {
> +    case 0x78:

It occurs to me (after spending far too long attempting to locate these
instructions in the manual), that we should consider where the cpuid
checks should go.

This opcode is particularly awkward.  Without a prefix, it is Intel's
VMREAD instruction, but with 66 or f2, it becomes part of AMD's SSE4A. 
(I guess this answers why the VT-x instructions strictly may not be
encoded with legacy prefixes.)

On real hardware, the cpuid check would logically be here, as it
indicates an inability of the pipeline to understand an instruction. 
OTOH, there is an argument to be made about sizing instructions which
are understood, but shouldn't be successfully emulated.

I don't think any of this is a blocker to the patch, so Reviewed-by:
Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, but we certainly should
consider it.

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