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Subject: Re: [PATCH 00/11] Use global pages with PTI
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From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <a3837d26-031f-6bce-cb1e-f34e5c0cfd2f@linux.intel.com>
Date: Tue, 27 Mar 2018 13:19:40 -0700
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To: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andrea Arcangeli <aarcange@redhat.com>, Andrew Lutomirski <luto@kernel.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>, the arch/x86 maintainers <x86@kernel.org>, namit@vmware.com

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.