linux-kernel - RE: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

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Date:   Wed, 18 Nov 2020 09:30:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexandre Chartre' <alexandre.chartre@...cle.com>,
        Borislav Petkov <bp@...en8.de>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "jroedel@...e.de" <jroedel@...e.de>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "jan.setjeeilers@...cle.com" <jan.setjeeilers@...cle.com>,
        "junaids@...gle.com" <junaids@...gle.com>,
        "oweisse@...gle.com" <oweisse@...gle.com>,
        "rppt@...ux.vnet.ibm.com" <rppt@...ux.vnet.ibm.com>,
        "graf@...zon.de" <graf@...zon.de>,
        "mgross@...ux.intel.com" <mgross@...ux.intel.com>,
        "kuzuno@...il.com" <kuzuno@...il.com>
Subject: RE: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

From: Alexandre Chartre
> Sent: 18 November 2020 07:42
> 
> 
> On 11/17/20 10:26 PM, Borislav Petkov wrote:
> > On Tue, Nov 17, 2020 at 07:12:07PM +0100, Alexandre Chartre wrote:
> >> Some benchmarks are available, in particular from phoronix:
> >
> > What I was expecting was benchmarks *you* have run which show that
> > perf penalty, not something one can find quickly on the internet and
> > something one cannot always reproduce her-/himself.
> >
> > You do know that presenting convincing numbers with a patchset greatly
> > improves its chances of getting it upstreamed, right?
> >
> 
> Well, it looks like I wrongfully assume that KPTI was a well known performance
> overhead since it was introduced (because it adds extra page-table switches),
> but you are right I should be presenting my own numbers.

IIRC the penalty comes from the page table switch.
Doing it at a different time is unlikely to make much difference.

For some workloads the penalty is massive - getting on for 50%.
We are still using old kernels on AWS.

	David

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