On Mon, 2018-08-20 at 15:27 -0700, Linus Torvalds wrote:
> On Mon, Aug 20, 2018 at 3:02 PM Woodhouse, David <dwmw@amazon.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.