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From:   Pavel Tatashin <pasha.tatashin@oracle.com>
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Subject: [PATCH v14 08/25] x86/kvmclock: Avoid TSC recalibration
Date:   Tue, 17 Jul 2018 22:21:54 -0400
Message-Id: <20180718022211.6259-9-pasha.tatashin@oracle.com>
In-Reply-To: <20180718022211.6259-1-pasha.tatashin@oracle.com>
References: <20180718022211.6259-1-pasha.tatashin@oracle.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: 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>
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
---
 arch/x86/kernel/kvmclock.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index ed170171fe49..da0ede8ac8f6 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -141,7 +141,16 @@ static inline void kvm_sched_clock_init(bool stable)
  */
 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)
-- 
2.18.0