Messages in this thread | | | Date | Tue, 16 Jan 2018 01:41:28 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI |
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* 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
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