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Date:   Tue, 27 Mar 2018 19:51:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Dave Hansen <dave.hansen@linux.intel.com>
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>,
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        Kees Cook <keescook@google.com>,
        Hugh Dickins <hughd@google.com>,
        =?ISO-8859-15?Q?J=FCrgen_Gro=DF?= <jgross@suse.com>,
        the arch/x86 maintainers <x86@kernel.org>, namit@vmware.com
Subject: Re: [PATCH 00/11] Use global pages with PTI
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On Tue, 27 Mar 2018, Dave Hansen wrote:

> On 03/27/2018 06:36 AM, Thomas Gleixner wrote:
> >>                         User Time       Kernel Time     Clock Elapsed
> >> Baseline ( 0 GLB PTEs)  803.79          67.77           237.30
> >> w/series (28 GLB PTEs)  807.70 (+0.7%)  68.07 (+0.7%)   238.07 (+0.3%)
> >>
> >> Without PCIDs, it behaves the way I would expect.
> > What's the performance benefit on !PCID systems? And I mean systems which
> > actually do not have PCID, not a PCID system with 'nopcid' on the command
> > line.
> 
> Do you have something in mind for this?  Basically *all* of the servers
> that I have access to have PCID because they are newer than ~7 years old.
> 
> That leaves *some* Ivybridge and earlier desktops, Atoms and AMD

AMD is not interesting as it's not PTI and uses GLOBAL anyway.

> systems.  Atoms are going to be the easiest thing to get my hands on,
> but I tend to shy away from them for performance work.

What I have in mind is that I wonder whether the whole circus is worth it
when there is no performance advantage on PCID systems.

Thanks,

	tglx