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

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Message-ID: <20180111193431.7e2d7f63@alans-desktop>
Date:   Thu, 11 Jan 2018 19:34:31 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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>, Willy Tarreau <w@....eu>,
        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, 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.

Alan