linux-kernel - [KVM timekeeping 33/35] Indicate reliable TSC in kvmclock

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Date:	Thu, 19 Aug 2010 22:07:47 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	kvm@...r.kernel.org
Cc:	Zachary Amsden <zamsden@...hat.com>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: [KVM timekeeping 33/35] Indicate reliable TSC in kvmclock

When no platform bugs have been detected, no TSC warps have been
detected, and the hardware guarantees to us TSC does not change
rate or stop with P-state or C-state changes, we can consider it reliable.

Signed-off-by: Zachary Amsden <zamsden@...hat.com>
---
 arch/x86/kvm/x86.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86f182a..a7fa24e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -55,6 +55,7 @@
 #include <asm/mce.h>
 #include <asm/i387.h>
 #include <asm/xcr.h>
+#include <asm/pvclock-abi.h>
 
 #define MAX_IO_MSRS 256
 #define CR0_RESERVED_BITS						\
@@ -900,6 +901,13 @@ static void kvm_get_time_scale(uint32_t scaled_khz, uint32_t base_khz,
 static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
 unsigned long max_tsc_khz;
 
+static inline int kvm_tsc_reliable(void)
+{
+	return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+		boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+		!check_tsc_unstable());
+}
+
 static inline u64 nsec_to_cycles(struct kvm *kvm, u64 nsec)
 {
 	return pvclock_scale_delta(nsec, kvm->arch.virtual_tsc_mult,
@@ -1151,7 +1159,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
 	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
 	vcpu->last_kernel_ns = kernel_ns;
-	vcpu->hv_clock.flags = 0;
+	vcpu->hv_clock.flags = kvm_tsc_reliable() ? PVCLOCK_TSC_STABLE_BIT : 0;
 
 	/*
 	 * The interface expects us to write an even number signaling that the
-- 
1.7.1

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