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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Date:   Mon, 20 Aug 2018 16:35:57 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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>, jsteckli@...inf.tu-dresden.de,
        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 Mon, Aug 20, 2018 at 03:27:52PM -0700, Linus Torvalds wrote:
> On Mon, Aug 20, 2018 at 3:02 PM Woodhouse, David <dwmw@...zon.co.uk> wrote:
> >
> > It's the *kernel* we don't want being able to access those pages,
> > because of the multitude of unfixable cache load gadgets.
> 
> Ahh.
> 
> I guess the proof is in the pudding. Did somebody try to forward-port
> that patch set and see what the performance is like?
> 
> It used to be just 500 LOC. Was that because they took horrible
> shortcuts? Are the performance numbers for the 32-bit case that
> already had the kmap() overhead?

The last version I worked on was a bit before Meltdown was public:
https://lkml.org/lkml/2017/9/7/445

The overhead was a lot, but Dave Hansen gave some ideas about how to
speed things up in this thread: https://lkml.org/lkml/2017/9/20/828

Since meltdown hit, I haven't worked seriously on understand and
implementing his suggestions, in part because it wasn't clear to me
what pieces of the infrastructure we might be able to re-use. Someone
who knows more about mm/ might be able to suggest an approach, though.

Tycho