prefer tsc timecounter when it's good

[Andriy Gapon]

Guys,

what do you think about the following change?
The idea is mark TSC as the best timecounter when it's invariant and synchronized
between cores.
Unfortunately I don't have code to auto-detect the synchronization and keep
relying on the corresponding tunable.  I thought about auto-setting it for
single-package configurations, but even that information is currently not trivial
to get out of our mp (i386/amd64) machdep code.

--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
@@ -169,6 +169,9 @@ init_TSC_tc(void)
 			printf("TSC timecounter disabled: APM enabled.\n");
 	}

+	if (tsc_is_invariant)
+		tsc_timecounter.tc_quality = 1200;
+
 #ifdef SMP
 	/*
 	 * We can not use the TSC in SMP mode unless the TSCs on all CPUs

-- 
Andriy Gapon