linux-kernel - Re: [PATCH v6 07/10] x86, mpx: decode MPX instruction to get bound violation information

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Date:	Thu, 19 Jun 2014 10:32:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	"Ren, Qiaowei" <qiaowei.ren@...el.com>,
	Borislav Petkov <bp@...en8.de>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 07/10] x86, mpx: decode MPX instruction to get bound
 violation information

On 06/19/2014 10:04 AM, Dave Hansen wrote:
> 
> Could you please support this position with some data?  I'm a bit
> skeptical that instruction decoding is going to be a
> performance-critical path.
> 
> I also don't see the extra field that you talked about in the previous
> thread?  What's the extra field?  I see a 'limit' vs. 'length', but you
> don't use 'length' at all, so I think you can use it instead, or at
> least union it.
> 
> I've taken a quick stab at trying to consolidate things.  I think I may
> have screwed up this:
> 
> 	insn->limit = MAX_MPX_INSN_SIZE - bytes;
> 
> Qiaowei, is there anything fundamentally broken with what I've got here?
> 

So I encouraged Qiaowei to do a limited special-purpose decoder, simply
because the glue to use the generic decoder was almost as large.  I am
overall not a huge fan of using the generic decoder in constrained
situation, because the generic decoder is very heavyweight not just in
terms of performance but in terms of interface -- because it has to.

	-hpa

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