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Date: Tue, 21 Aug 2018 10:57:03 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: 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, 2018-08-20 at 15:27 -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?
I hadn't actually seen the XPFO patch set before; we're going to take a
serious look.
Of course, this is only really something that a select few people (with
quite a lot of machines) would turn on. And they might be willing to
tolerate a significant performance cost if the alternative way to be
safe is to disable hyperthreading entirely — which is Intel's best
recommendation so far, it seems.
Another alternative... I'm told POWER8 does an interesting thing with
hyperthreading and gang scheduling for KVM. The host kernel doesn't
actually *see* the hyperthreads at all, and KVM just launches the full
set of siblings when it enters a guest, and gathers them again when any
of them exits. That's definitely worth investigating as an option for
x86, too.
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