x86/tsc: Try to adjust TSC if sync test fails

If the first CPU of a package comes online, it is necessary to test whether
the TSC is in sync with a CPU on some other package. When a deviation is
observed (time going backwards between the two CPUs) the TSC is marked
unstable, which is a problem on large machines as they have to fall back to
the HPET clocksource, which is insanely slow.

It has been attempted to compensate the TSC by adding the offset to the TSC
and writing it back some time ago, but this never was merged because it did
not turn out to be stable, especially not on older systems.

Modern systems have become more stable in that regard and the TSC_ADJUST
MSR allows us to compensate for the time deviation in a sane way. If it's
available allow up to three synchronization runs and if a time warp is
detected the starting CPU can compensate the time warp via the TSC_ADJUST
MSR and retry. If the third run still shows a deviation or when random time
warps are detected the test terminally fails.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/20161119134018.048237517@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

0 files changed, 0 insertions, 0 deletions