linux-kernel - Re: [PATCH 1/2] x86: insn decoder: create artificial 3rd byte for 2-byte VEX

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Date:	Sun, 18 May 2014 00:59:54 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org, "Frank Ch. Eigler" <fche@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 1/2] x86: insn decoder: create artificial 3rd byte for
 2-byte VEX

(2014/05/17 3:34), Denys Vlasenko wrote:
> Before this patch, users need to do this to fetch vex.vvvv:
> 
>         if (insn->vex_prefix.nbytes == 2) {
>                 vex_vvvv = ((insn->vex_prefix.bytes[1] >> 3) & 0xf) ^ 0xf;
>         }
>         if (insn->vex_prefix.nbytes == 3) {
>                 vex_vvvv = ((insn->vex_prefix.bytes[2] >> 3) & 0xf) ^ 0xf;
>         }
> 
> Make it so that insn->vex_prefix.bytes[2] always contains vex.wvvvvLpp bits.

I like this hack :)
If you don't mind, please add inline functions to get such vex bits from
struct insn too?

Thank you,

> 
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Cc: Frank Ch. Eigler <fche@...hat.com>
> Cc: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> Cc: Ananth N Mavinakayanahalli <ananth@...ibm.com>
> Cc: Jim Keniston <jkenisto@...ux.vnet.ibm.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Andi Kleen <andi@...stfloor.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---
>  arch/x86/lib/insn.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 54fcffe..829ca4c 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c
> @@ -163,6 +163,12 @@ found:
>  				/* VEX.W overrides opnd_size */
>  				insn->opnd_bytes = 8;
>  		} else {
> +			/*
> +			 * For VEX2, fake VEX3-like byte#2.
> +			 * Makes it easier to decode vex.W, vex.vvvv,
> +			 * vex.L and vex.pp. Masking with 0x7f sets vex.W == 0.
> +			 */
> +			insn->vex_prefix.bytes[2] = b2 & 0x7f;
>  			insn->vex_prefix.nbytes = 2;
>  			insn->next_byte += 2;
>  		}
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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