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

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim Krcmar <rkrcmar@redhat.com>,
	Juergen Gross <jgross@suse.com>,
	Pavel Tatashin <pasha.tatashin@oracle.com>,
	steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
	x86@kernel.org, kvm@vger.kernel.org
Subject: [patch 8/7] x86/kvmclock: Avoid TSC recalibration
Date: Mon, 9 Jul 2018 11:22:43 +0200	[thread overview]
Message-ID: <20180709092243.GG2476@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180706161307.733337643@linutronix.de>


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)
