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SubjectRe: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization
  On 09/14/2010 11:10 AM, Jan Kiszka wrote:
> Am 20.08.2010 10:07, 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 | 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;
> For yet unknown reason, this commit breaks Linux guests here if they are
> started with only a single VCPU. They hang during boot, obviously no
> longer receiving interrupts.
>
> I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a side
> effect of the wrapping, though I cannot imagine how.
>
> Anyone any ideas?
>
>

Most likely, time went backwards, and some 'future - past' calculation
resulted in a negative sleep value which was then interpreted as
unsigned and resulted in a 2342525634 year sleep.

Does your guest use kvmclock, tsc, or some other time source?

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



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