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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 <feng.tang@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <x86@kernel.org>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
a7ec817d55