linux-kernel - Re: [PATCH 2/2] x86: extend insn decoder to understand xop and evex prefixes

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Message-ID: <537C5868.7020302@hitachi.com>
Date:	Wed, 21 May 2014 16:40:24 +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 2/2] x86: extend insn decoder to understand xop and evex
 prefixes

(2014/05/20 0:04), Denys Vlasenko wrote:
> On 05/17/2014 06:00 PM, Masami Hiramatsu wrote:
>> (2014/05/17 3:34), Denys Vlasenko wrote:
>>> Since xop and evex prefixes are extensions of vex mechanism,
>>> they have similar bit layouts, and they can never be combined
>>> (an instruction can have only one of them),
>>> (ab)use insn->vex_prefix to store data of xop and evex too.
>>>
>>> Users will need to conditionalize on insn->vex_prefix.bytes[0]
>>> instead of insn->vex_prefix.nbytes if they want to determine
>>> which of vex(-like) prefixes are there.
>>>
>>> Instead of adding more inattr bits for these prefixes, drop
>>> VEX inattr bits and use VEX opcode values directly to detect them.
>>> There is no point in having additional level of indirection here.
>>> (And we are close to running out of inattr bits for prefixes).
>>
>> Thank you very much for trying this work :)
>> But sorry, Nak, I don't like to use the prefix byte directly.
>> I'd rather like to add additional inattr bits for them.
> 
> Ok, I'm looking at inat.h and looks like I need to widen
> INAT_PFX_BITS, otherwise I have no space for two more bits
> for two more prefixes (XOP and EVEX):

Yes, that's what I look forward! :)

> 
> /* AVX VEX prefixes */
> #define INAT_PFX_VEX2   13      /* 2-bytes VEX prefix */
> #define INAT_PFX_VEX3   14      /* 3-bytes VEX prefix */
> ...
> 
> /* Legacy prefix */
> #define INAT_PFX_OFFS   0
> #define INAT_PFX_BITS   4
> #define INAT_PFX_MAX    ((1 << INAT_PFX_BITS) - 1)
> #define INAT_PFX_MASK   (INAT_PFX_MAX << INAT_PFX_OFFS)
> 
> 


-- 
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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