The introduction of clocksource_tsc_early broke the functionality of
"tsc=reliable" and "tsc=nowatchdog" command line parameters, since
clocksource_tsc_early is unconditionally registered with
CLOCK_SOURCE_MUST_VERIFY and thus put on the watchdog list.

This can cause the TSC to be declared unstable during boot:

  clocksource: timekeeping watchdog on CPU0: Marking clocksource
               'tsc-early' as unstable because the skew is too large:
  clocksource: 'refined-jiffies' wd_now: fffb7018 wd_last: fffb6e9d
               mask: ffffffff
  clocksource: 'tsc-early' cs_now: 68a6a7070f6a0 cs_last: 68a69ab6f74d6
               mask: ffffffffffffffff
  tsc: Marking TSC unstable due to clocksource watchdog

The corresponding elapsed times are cs_nsec=1224152026 and wd_nsec=378942392, so
the watchdog differs from TSC by 0.84 seconds.

This happens when HPET is not available and jiffies are used as the TSC
watchdog instead and the jiffies update is not happening due to lost timer
interrupts in periodic mode, which can happen e.g. with expensive debug
mechanisms enabled or under massive overload conditions in virtualized
environments.

Before the introduction of the early TSC clocksource the command line
parameters "tsc=reliable" and "tsc=nowatchdog" could be used to work around
this issue.

Restore the behaviour by disabling the watchdog if requested on the kernel
command line.

[ tglx: Clarify changelog ]

Fixes: aa83c457 ("x86/tsc: Introduce early tsc clocksource")
Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191024175945.14338-1-mzhivich@akamai.com