Re: [PATCH 04/17] Fix deep C-state TSC desynchronization

Avi Kivity
2010-06-15T08:09:35Z

On 06/15/2010 10:34 AM, 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<zamsden@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;
> }
>

That will cause us to miss a vmclear later on. Also it invalidates an
invariant that the per-cpu list vcpus_on_cpu has all the vcpus with
vcpu->cpu == cpu on this cpu (try saything that fast).

--
error compiling committee.c: too many arguments to function