Date: Thu, 11 Jan 2018 23:06:59 +0100
From: Willy Tarreau <w@1wt.eu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Olivier Galibert <galibert@pobox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set
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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 :-)

Willy