Messages in this thread | | | Date | Mon, 21 Jul 2014 23:35:34 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86, TSC: Add a software TSC offset |
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On Mon, Jul 21, 2014 at 12:34:40PM -0700, Andy Lutomirski wrote: > I'm sure I'm missing something, but where is tsc_offset used outside > the tsc startup code? Is it somehow getting programmed into the TSC > config registers?
Nah, we're programming the CPU number into the TSC_AUX MSR. However, that approach might become obsolete for a simpler, preempt-disable version which Peter suggested today. It should be comparatively cheap and work on all CPUs, not only RDTSCP-supporting ones:
get_cycles() { preempt_disable(); rdtscll() + this_cpu_read(tsc_offset); preempt_disable() }
So, to answer your question, the TSC offset will be used in get_cycles(). So even on machines with a skewed TSC, we will have a stable TSC counter by adding the per-cpu offset each time we read the TSC.
We'll see how it plays out in testing. :)
-- Regards/Gruss, Boris.
Sent from a fat crate under my desk. Formatting is fine. --
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