clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin
Right now, cs_watchdog_read() does clocksource sanity checks based
on WATCHDOG_MAX_SKEW, which sets a floor on any clocksource's
.uncertainty_margin. These sanity checks can therefore act
inappropriately for clocksources with large uncertainty margins.
One reason for a clocksource to have a large .uncertainty_margin is when
that clocksource has long read-out latency, given that it does not make
sense for the .uncertainty_margin to be smaller than the read-out latency.
With the current checks, cs_watchdog_read() could reject all normal
reads from a clocksource with long read-out latencies, such as those
from legacy clocksources that are no longer implemented in hardware.
Therefore, recast the cs_watchdog_read() checks in terms of the
.uncertainty_margin values of the clocksources involved in the timespan
in question. The first covers two watchdog reads and one cs read,
so use twice the watchdog .uncertainty_margin plus that of the cs.
The second covers only a pair of watchdog reads, so use twice the
watchdog .uncertainty_margin.
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.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: Peter Zijlstra <peterz@infradead.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Cc: <x86@kernel.org>
1 file changed
