linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Date:   Thu, 11 Jan 2018 22:23:48 +0100
From:   Willy Tarreau <w@....eu>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Olivier Galibert <galibert@...ox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On Thu, Jan 11, 2018 at 07:34:31PM +0000, Alan Cox wrote:
> On Thu, 11 Jan 2018 13:26:01 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > On Thu, Jan 11, 2018 at 08:19:35PM +0100, Olivier Galibert wrote:
> > > Wouldn't the time taken by an easy syscall like getuid be a clear indicator?  
> > 
> > I think only if you had a baseline measurement to compare against.
> 
> getuid can also be cached by a smart libc. getppid() is what micro
> benchmarks seem to favour.

I initially tried getpid() which I found to be cached by glibc, but I
switched to write(-1, "a", 1) in the example in the cover letter and
the test for 3 million runs roughly climbs from 200 ms to 900 ms under
kvm (with PCID this time, I didn't retest without).

Willy