[MODERATED] Re: [PATCH 1/1] SMTDOCv2 0

From: Andi Kleen <ak@linux.intel.com>
To: speck@linutronix.de
Subject: [MODERATED] Re: [PATCH 1/1] SMTDOCv2 0
Date: Sat, 7 Jul 2018 20:45:11 -0700	[thread overview]
Message-ID: <20180708034511.GL25550@tassilo.jf.intel.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1807071058120.1589@nanos.tec.linutronix.de>

On Sat, Jul 07, 2018 at 11:39:02AM +0200, speck for Thomas Gleixner wrote:
> On Fri, 6 Jul 2018, speck for Andi Kleen wrote:
> > --- /dev/null
> > +++ b/Documentation/virtual/kvm/l1tf-mitigation.txt
> 
> I seem to remember that I asked for RST format. Aside of that we really

I probably lost that somewhere in your flood of ad hominems.

> > @@ -0,0 +1,69 @@
> > +L1TF mitigation strategies in KVM
> > +On some Intel CPUs a side channel attack may make it possible to read
> 
> On some? L1TF affects every fricking Intel CPU with EPT except a few ATOMs.

Think what will the state be when someone reads this in two years.

> 
> > +arbitrary data in the L1D cache of the CPU core in the guest, potentially allowing
> > +to read data from outside the VM.
> 
> This really wants to be explained so that the admin who is supposed to read
> that understands the mechanims at least at the high level. That
> 'may/potentially' weasel wording is not at all helpful to make decisions
> about the mitigations.

There will be an Intel white paper with details, we can point to that later.
I don't think going into micro architectural details here will
help any admin.

> 
> Fails? How should someone who is not familiar with the deep details of that
> decided that the above fails?

It's a list of actions all with preconditions. For each it is clear
if you can or cannot do the action. 

> 
> > +
> > +SMT improves performance so it should be only disabled when absolutely
> 
> SMT improves performance depending on the workload. There are workloads
> which are negatively impacted by SMT.
> 
> That said, this all wants to be properly structured and explained in
> detail. A hastily cobbled together string of syllables does not make a
> document which is intended to provide guidance to people who do not have a
> deep technical insight into this.

We can perhaps expand some things slightly.

But I doubt any admin is interested in reading a novel on this here. I
certainly wouldn't be. There will be plenty of other material elsewhere for
people who want all the gory details.

-Andi