linux-kernel - Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

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Message-ID: <CA+55aFzHj_GNZWG4K2oDu4DPP9sZdTZ9PY7sBxGB6WoN9g8d=A@mail.gmail.com>
Date:   Sat, 1 Sep 2018 14:38:43 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     jsteckli@...zon.de
Cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        juerg.haefliger@....com, deepa.srinivasan@...cle.com,
        Jim Mattson <jmattson@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        joao.m.martins@...cle.com, pradeep.vincent@...cle.com,
        Andi Kleen <ak@...ux.intel.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        kanth.ghatraju@...cle.com, Liran Alon <liran.alon@...cle.com>,
        Kees Cook <keescook@...gle.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        chris.hyser@...cle.com, Tyler Hicks <tyhicks@...onical.com>,
        John Haxby <john.haxby@...cle.com>,
        Jon Masters <jcm@...hat.com>
Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)

On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@...zon.de> wrote:
>
> I've been spending some cycles on the XPFO patch set this week. For the
> patch set as it was posted for v4.13, the performance overhead of
> compiling a Linux kernel is ~40% on x86_64[1]. The overhead comes almost
> completely from TLB flushing. If we can live with stale TLB entries
> allowing temporary access (which I think is reasonable), we can remove
> all TLB flushing (on x86). This reduces the overhead to 2-3% for
> kernel compile.

I have to say, even 2-3% for a kernel compile sounds absolutely horrendous.

Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
from a kernel is not some small unnoticeable thing.

           Linus