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    SubjectRE: [PATCH] x86/copy_user_generic: Optimize copy_user_generic with CPU erms feature
    Date
    > From: David Miller [mailto:davem@davemloft.net]
    > Sent: Thursday, May 24, 2012 6:50 PM
    > From: "Fenghua Yu" <fenghua.yu@intel.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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