[MODERATED] Re: [PATCH 1/1 v2] Linux patch #1

[Andi Kleen <ak@linux.intel.com>]

> Agreed; thinking about this a little bit more from admin POV, I think we 
> should just go with
> 
> (1) l1tf=full
>     That would currently mean PTEINV + nosmt=force, and later once the 
>     KVM/Xen bits land in, they will make use of it as well (KVM will start 
>     doing the flushes (*), Xen doing the PTE sanitization or their variant 
>     of gang scheduling or whatnot)

That means everyone who doesn't use KVM or uses KVM in a way that is
safe wastes 30%+ of performance completely unnecessarily.

I don't think this is a good idea. Everyone who disables SMT should
make a conscious decision on this. 

L1TF is complex and "turn off your brain, just use big hammer" is not
a suitable strategy for it.

KVM already prints warnings when SMT is on, this can be perhaps
made more clear. 

The decision tree for starting KVM with SMT on is roughly:

- If you trust your guests do nothing
- If your KVM is in a cpuset or affinity that has its own set of
cores do nothing (we usually assume interrupts leaking are not
a problem)
- Then first try to bind the KVM to a subset of cores and only
turn off SMT for those
- If that all fails turn off SMT globally. This should be an
explicit option, not lumped in with something else.

-Andi