Intel CPU design flaw - FreeBSD affected? // disabling LDTSC

[Klaus P. Ohrhallinger]

On 04.01.2018 19:51, Jan Kokemüller wrote:

> It is possible to emulate a high resolution counter with a thread that
> continuously increments a variable [1]. This is the reason why browser
> vendors are currently disabling the SharedArrayBuffer feature [2].
> 
> [1]: https://gist.github.com/ErikAugust/724d4a969fb2c6ae1bbd7b2a9e3d4bb6#gistcomment-2311156
> [2]: https://blog.mozilla.org/security/2018/01/03/mitigations-landing-new-class-timing-attack/

I tried the phtread example from [1] but even with some tweaking is does
not work at all.

This is a multiprocessor system, with moderate load.

As far as I understand the matter, it can only work if both threads
share the same cpu cache, otherwise the counter variable is either never
up-to-date, or has to be fetched and stored from/to memory, which is way
too slow for this purpose.

Any suggestions ?

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CPU: Intel(R) Xeon(R) CPU           E5420  @ 2.50GHz (2500.14-MHz
K8-class CPU)
FreeBSD/SMP: Multiprocessor System Detected: 8 CPUs
FreeBSD/SMP: 2 package(s) x 4 core(s)