lkml.org 
[lkml]   [2010]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization
Am 14.09.2010 11:27, Avi Kivity wrote:
> 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.

Looks like that's the case on first glance at the apic state.

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

A kernel that has kvmclock support even hangs in SMP mode. The others
pick hpet or acpi_pm. TSC is considered unstable.

Jan

--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux


\
 
 \ /
  Last update: 2010-09-14 12:43    [W:0.089 / U:0.216 seconds]
©2003-2014 Jasper Spaans. Advertise on this site