Re: [PATCH v8 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource

On Mon, 22 Feb 2016, Christopher S. Hall wrote:

On modern Intel systems TSC is derived from the new Always Running Timer
(ART). ART can be captured simultaneous to the capture of
audio and network device clocks, allowing a correlation between timebases
to be constructed. Upon capture, the driver converts the captured ART
value to the appropriate system clock using the correlated clocksource
mechanism.

On systems that support ART a new CPUID leaf (0x15) returns parameters
“m” and “n” such that:

TSC_value = (ART_value * m) / n + k [n >= 2]

[k is an offset that can adjusted by a privileged agent. The
IA32_TSC_ADJUST MSR is an example of an interface to adjust k.
See 17.14.4 of the Intel SDM for more details]

Signed-off-by: Christopher S. Hall <redacted>
[jstultz: Tweaked to fix build issue, also reworked math for
64bit division on 32bit systems]
Signed-off-by: John Stultz <redacted>

Reviewed-by: Thomas Gleixner <redacted>