Re: [Qemu-devel] [PATCH RFC] KVM: X86: save and load PVCLOCK_TSC_UNSTABLE_BIT when migration

[Jay Zhou <jianjay.zhou@huawei.com>]

Hi Radim,

On 2017/6/13 3:52, Radim Krčmář wrote:
> 2017-06-12 21:23+0800, Jay Zhou:
>> Guest using kvmclock will be hanged when migrating from unstable
>> tsc host to stable tsc host occasionally.
>> Sometimes, the tsc timestamp saved at the source side will be
>> backward when the guest stopped, and this value is transferred
>> to the destination side. The guest at the destination side thought
>> kvmclock is stable, so the protection mechanism against time
>> going backwards is not used.
>> When the first time vcpu0 enters the guest at the destination
>> side to update the wall clock, the result of
>> pvclock_clocksource_read will be backward occasionally,
>> which results in the wall clock drift.
>>
>> Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
>> ---
>
> Hm, are you using KVM that has e3fd9a93a12a (4.9+)?

I'm using KVM (4.4.11) and QEMU (2.6.0). I thought the newest KVM
and QEMU have the same problem, maybe I'm wrong.

>
> If you get a timestamp from KVM_GET_CLOCK() and pass that to
> KVM_SET_CLOCK(), then kvmclock should not jump backwards anymore
> (it could before 4.9, but only if the host had stable tsc).

If migrating from old KVMs to newer KVMs (v4.9+), the clock member
of struct KVMClockState is migrated to the destination side, but the
clock_is_reliable isn't migrated. The value of clock_is_reliable is false
at the source side, and it is true at the destination side. Is it possible
the clock member of struct KVMClockState migrated is smaller than the
last_value saved in the guest theoretically? Could you explain more
why it should not jump backwards in the scenario of migrating from unstable tsc 
host to stable tsc host?

>
> A possible source of this bug in when QEMU recomputes the timestamp to
> pass to KVM_SET_CLOCK() using TSC and kvmclock page.

Yes, this bug happened in this scenario in my tests.

>
> Can you provide the values used for KVM_GET_CLOCK and KVM_SET_CLOCK when
> the bug occurs?

I didn't print these values out when the bug occured, and I do not have the 
physical machine with unstable tsc clocksource now. But I will try to reproduce 
with the newest KVM and QEMU once I have.


Regards,
Jay Zhou