From a7ec817d55421ac214cac9d3e5ebb65d848198dc Mon Sep 17 00:00:00 2001 From: Feng Tang Date: Wed, 4 Jan 2023 16:19:38 +0800 Subject: x86/tsc: Add option to force frequency recalibration with HW timer The kernel assumes that the TSC frequency which is provided by the hardware / firmware via MSRs or CPUID(0x15) is correct after applying a few basic consistency checks. This disables the TSC recalibration against HPET or PM timer. As a result there is no mechanism to validate that frequency in cases where a firmware or hardware defect is suspected. And there was case that some user used atomic clock to measure the TSC frequency and reported an inaccuracy issue, which was later fixed in firmware. Add an option 'recalibrate' for 'tsc' kernel parameter to force the tsc freq recalibration with HPET or PM timer, and warn if the deviation from previous value is more than about 500 PPM, which provides a way to verify the data from hardware / firmware. There is no functional change to existing work flow. Recently there was a real-world case: "The 40ms/s divergence between TSC and HPET was observed on hardware that is quite recent" [1], on that platform the TSC frequence 1896 MHz was got from CPUID(0x15), and the force-reclibration with HPET/PMTIMER both calibrated out value of 1975 MHz, which also matched with check from software 'chronyd', indicating it's a problem of BIOS or firmware. [Thanks tglx for helping improving the commit log] [ paulmck: Wordsmith Kconfig help text. ] [1]. https://lore.kernel.org/lkml/20221117230910.GI4001@paulmck-ThinkPad-P17-Gen-1/ Signed-off-by: Feng Tang Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" Cc: Jonathan Corbet Cc: Cc: Signed-off-by: Paul E. McKenney --- Documentation/admin-guide/kernel-parameters.txt | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'Documentation/admin-guide') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 6cfa6e3996cf..95f0d104c232 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -6369,6 +6369,10 @@ in situations with strict latency requirements (where interruptions from clocksource watchdog are not acceptable). + [x86] recalibrate: force recalibration against a HW timer + (HPET or PM timer) on systems whose TSC frequency was + obtained from HW or FW using either an MSR or CPUID(0x15). + Warn if the difference is more than 500 ppm. tsc_early_khz= [X86] Skip early TSC calibration and use the given value instead. Useful when the early TSC frequency discovery -- cgit v1.2.3-58-ga151