On Mon, Jul 21, 2014 at 2:35 PM, Borislav Petkov <[email protected]> wrote: > 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. :)
How will this be compatible with the vdso? Also, IIRC, rdtscp does not need rdtsc_barrier(), whereas rdtsc does. Getting this wrong will be a significant slowdown. --Andy > > -- > Regards/Gruss, > Boris. > > Sent from a fat crate under my desk. Formatting is fine. > -- -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
