linux-kernel - Re: [PATCH 00/11] Use global pages with PTI

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Message-ID: <a3837d26-031f-6bce-cb1e-f34e5c0cfd2f@linux.intel.com>
Date:   Tue, 27 Mar 2018 13:19:40 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jürgen Groß <jgross@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>, namit@...are.com
Subject: Re: [PATCH 00/11] Use global pages with PTI

On 03/27/2018 01:07 PM, Ingo Molnar wrote:
>  - To see at minimum stddev numbers, to make sure we are not looking at some weird
>    statistical artifact. (I also outlined a more robust measurement method.)
> 
>  - If the numbers are right, a CPU engineer should have a look if possible, 
>    because frankly this effect is not expected and is not intuitive. Where global 
>    pages can be used safely they are almost always an unconditional win.
>    Maybe we are missing some limitation or some interaction with PCID.
> 
> Since we'll be using PCID even on Meltdown-fixed hardware, maybe the same negative 
> performance effect already exists on non-PTI kernels as well, we just never 
> noticed?

Yep, totally agree.  I'll do the more robust collection and also explore
on "real" !PCID hardware.  I also know the right CPU folks to go ask
about this, I just want to do the second round of robust data collection
before I bug them.