[patch 8/7] x86/kvmclock: Avoid TSC recalibration

[Peter Zijlstra <peterz@infradead.org>]

If the host gives us a TSC rate, assume it is good and don't try and
recalibrate things against virtual timer hardware.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -140,7 +140,16 @@ static inline void kvm_sched_clock_init(
  */
 static unsigned long kvm_get_tsc_khz(void)
 {
-	return pvclock_tsc_khz(this_cpu_pvti());
+	unsigned long tsc_khz = pvclock_tsc_khz(this_cpu_pvti());
+
+	/*
+	 * TSC frequency is reported by the host; calibration against (virtual)
+	 * HPET/PM-timer in a guest is dodgy and pointless since the host already
+	 * did it for us where required.
+	 */
+	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
+	return tsc_khz;
 }
 
 static void kvm_get_preset_lpj(void)