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

Zachary Amsden
2010-06-15T08:14:02Z

On 06/14/2010 10:09 PM, Avi Kivity wrote:
> 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).
>

Blasted SVM / VMX differences! There are other, less elegant ways to
accomplish this task however.

Good catch, BTW.