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From: David Laight <David.Laight@ACULAB.COM>
To: 'Willy Tarreau' <w@1wt.eu>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
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Subject: RE: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set
Thread-Topic: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set
Thread-Index: AQHTiyi0p/EI9VTJykW3F/Uh2c3tDqNwcDwA
Date: Fri, 12 Jan 2018 16:37:54 +0000
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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