Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI

[Ingo Molnar <mingo@kernel.org>]

* Nadav Amit <nadav.amit@gmail.com> wrote:

> > Also, what's the end goal here?  Run old 32-bit binaries better?  You
> > want to weaken the security of the whole implementation to do that?
> > Sounds like a bad tradeoff to me.
> 
> As Willy noted in this thread, I think that some users may be interested in 
> running 32-bit Apache/Nginx/Redis to get the performance back without 
> sacrificing security.

Note that it is a flawed assumption to think that this is possible, as they might 
in many cases not be getting their performance back: 32-bit binaries for the same 
general CPU bound computation can easily be 5% slower than 64-bit binaries (as 
long as the larger cache footprint of 64-bit data doesn't fall out of key caches), 
but can be up to 30% slower for certain computations.

In fact, depending on how kernel heavy the web workload is (for example how much 
CGI processing versus IO it does, etc.), a 32-bit binary could be distinctly 
_slower_ than even a PTI-enabled 64-bit binary.

So we are trading a 5-15% slowdown (PTI) for another 5-15% slowdown, plus we are
losing the soft-SMEP feature on older CPUs that PTI enables, which is a pretty 
powerful mitigation technique.

Yes, I suspect in some (maybe many) cases it would be a speedup, but I really 
don't like the underlying assumptions and tradeoffs here. (Not that I like any of 
this whole Meltdown debacle TBH.)

Thanks,

	Ingo