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From: Zachary Amsden on 15 Jun 2010 03:40
When CPUs with unstable TSCs enter deep C-state, TSC may stop
running. This causes us to require resynchronization. Since
we can't tell when this may potentially happen, we assume the
worst by forcing re-compensation for it at every point the VCPU
task is descheduled.

Signed-off-by: Zachary Amsden <zamsden(a)redhat.com>
---
arch/x86/kvm/x86.c | 11 +++++++++++
1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c8289d0..618c435 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
{
kvm_x86_ops->vcpu_put(vcpu);
kvm_put_guest_fpu(vcpu);
+
vcpu->arch.last_host_tsc = native_read_tsc();
+
+ /*
+ * When potentially leaving a CPU with unstable TSCs, we risk
+ * that the CPU enters deep C-state. If it does, the TSC may
+ * go out of sync but we will not recalibrate because the test
+ * vcpu->cpu != cpu can not detect this condition. So set
+ * vcpu->cpu = -1 to force the recalibration above.
+ */
+ if (check_tsc_unstable())
+ vcpu->cpu = -1;
}

static int is_efer_nx(void)
--
1.7.1

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Prev: [PATCH 01/17] Eliminate duplicated timer code
Next: [PATCH 10/17] Make KVM clock computation work for other scales