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

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Message-ID: <537C5812.6070002@hitachi.com>
Date:	Wed, 21 May 2014 16:38:58 +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: Re: [PATCH 1/2] x86: insn decoder: create artificial 3rd byte
 for 2-byte VEX

(2014/05/19 23:58), Denys Vlasenko wrote:
> On 05/17/2014 05:59 PM, Masami Hiramatsu wrote:
>> (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?
> 
> I prefer to add such functionality when the first user surfaces:
> the needs of the user often dictate more suitable API.

OK, then this should be postponed until the actual user (uprobes)
appears.

> In this case (VEX insns), the API isn't that straightforward.
> For example, uprobe code narrowly escaped the need to detect maskmovq__
> (0f f7) and maskmovdqu (66 0f f7 or equivalent VEX) instructions.
> 
> Let's assume we do need to detect them.
> Merely fetching and looking at vex.mmmmm, vex.pp and opcode doesn't
> really help that much in detecting these instructions:
> with such simplistic help, user needs to check both VEX and legacy forms.
> 
> Perhaps a better API would be to form a word-sized "expanded opcode"
> for vector and VEX insns. Something along the lines of:
> 
> [x.mmmmm.pp.opcode__]
> 
> where x bit indicates XOP insn (as opposed to VEX), pp indicate
> none/66/F2/F3 prefixes, opcode__ is 8-bit opcode.
> then checking for maskmovFOO insns would be easier.

I see, but it also makes things complex just for special
instructions. I'd like to let decoder just decode it, not
elaborate.

Thank you,

-- 
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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