linux-kernel - RE: [PATCH] x86/copy_user_generic: Optimize copy_user

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Date:	Fri, 25 May 2012 02:47:22 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	David Miller <davem@...emloft.net>
CC:	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH] x86/copy_user_generic: Optimize copy_user_generic with
 CPU erms feature

> From: David Miller [mailto:davem@...emloft.net]
> Sent: Thursday, May 24, 2012 6:50 PM
> From: "Fenghua Yu" <fenghua.yu@...el.com>
> Date: Thu, 24 May 2012 18:19:45 -0700
> 
> > According to Intel 64 and IA-32 SDM and Optimization Reference
> > Manual, beginning with Ivybridge, REG string operation using MOVSB
> > and STOSB can provide both flexible and high-performance REG string
> > operations in cases like memory copy.  Enhancement availability is
> > indicated by CPUID.7.0.EBX[9] (Enhanced REP MOVSB/ STOSB).
> 
> How does the cpu do overlap detection?
> 
> If the cpu does overlap detection on sub-pagesize bits, performance
> will unnecessarily suffer under such circumstances.

Are you talking about memory overlap between source and destination? There is no overlap between these two areas in copy_user case because one area is in user space and another one is in kernel space.

In overlap case, it's software that detects overlap and sets backward copy. I don't see backward rep movsb performance degradation from my measurement.

Thanks.

-Fenghua
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