2010-08-20 08:15:07

by Zachary Amsden

Subject: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

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 <[email protected]>
---
arch/x86/kvm/x86.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7fc4a55..52b6c21 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
}

kvm_x86_ops->vcpu_load(vcpu, cpu);
- if (unlikely(vcpu->cpu != cpu)) {
+ if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
/* Make sure TSC doesn't go backwards */
s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
native_read_tsc() - vcpu->arch.last_host_tsc;
--
1.7.1

2010-08-20 17:30:51

by Glauber Costa

Subject: Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

On Thu, Aug 19, 2010 at 10:07:24PM -1000, Zachary Amsden wrote:
> 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 <[email protected]>
Fair enough.