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    SubjectRE: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti_disable is set
    Date
    From: Willy Tarreau
    > Sent: 11 January 2018 22:07
    > On Thu, Jan 11, 2018 at 01:28:18PM -0800, Linus Torvalds wrote:
    > > The traditional fast system call to test is getppid().
    > >
    > > write() goes through a lot more code.
    >
    > Just tried getppid() now, it's relatively similar (slightly slower than
    > write(-1) though, maybe that one aborts very early) :
    >
    > PTI=on : 920ms for 3 million calls
    > PTI=off (prctl) : 230ms for 3 million calls
    > PTI=off (boot) : 215ms for 3 million calls
    >
    > The small difference between the last two very likely comes from the few
    > instructions avoided thanks to the alternatives when pti=off is used at
    > boot.
    >
    > So yes here it's trivial to tell if it's on or off :-)

    A system call with a larger kernel memory footprint, and user
    code that touches more pages, might show an even bigger difference
    between PTI=on and PTI=off.

    David

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