There are cases that the TSC clocksource is wrongly judged as unstable by
the clocksource watchdog mechanism which tries to validate the TSC against
HPET, PM_TIMER or jiffies. While there is hardly a general reliable way to
check the validity of a watchdog, Thomas Gleixner proposed [1]:

"I'm inclined to lift that requirement when the CPU has:

    1) X86_FEATURE_CONSTANT_TSC
    2) X86_FEATURE_NONSTOP_TSC
    3) X86_FEATURE_NONSTOP_TSC_S3
    4) X86_FEATURE_TSC_ADJUST
    5) At max. 4 sockets

 After two decades of horrors we're finally at a point where TSC seems
 to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST
 was really key as we can now detect even small modifications reliably
 and the important point is that we can cure them as well (not pretty
 but better than all other options)."

As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations
of Atom processorz, and is always coupled with X86_FEATURE_CONSTANT_TSC
and X86_FEATURE_NONSTOP_TSC, skip checking it, and also be more defensive
to use maximal 2 sockets.

The check is done inside tsc_init() before registering 'tsc-early' and
'tsc' clocksources, as there were cases that both of them had been
wrongly judged as unreliable.

For more background of tsc/watchdog, there is a good summary in [2]

[tglx} Update vs. jiffies:

  On systems where the only remaining clocksource aside of TSC is jiffies
  there is no way to make this work because that creates a circular
  dependency. Jiffies accuracy depends on not missing a periodic timer
  interrupt, which is not guaranteed. That could be detected by TSC, but as
  TSC is not trusted this cannot be compensated. The consequence is a
  circulus vitiosus which results in shutting down TSC and falling back to
  the jiffies clocksource which is even more unreliable.

[1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@nanos.tec.linutronix.de/
[2]. https://lore.kernel.org/lkml/87a6pimt1f.ffs@nanos.tec.linutronix.de/

[ tglx: Refine comment and amend changelog ]

Fixes: 6e3cd952 ("x86/hpet: Use another crystalball to evaluate HPET usability")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211117023751.24190-2-feng.tang@intel.com