kvm live migration reliable?

kvm live migration reliable?

[Kent Tong]

Hi,

I was wondering the kvm live migration function is reliable? I read that if
the TSC is not monotonically increasing, then the guest will hang. How to
prevent this?

Thanks!

Re: kvm live migration reliable?

[Stephane Bakhos]

> I was wondering the kvm live migration function is reliable? I read that if
> the TSC is not monotonically increasing, then the guest will hang. How to
> prevent this?

No idea about that problem.
For migration, it usually works for me, but the clock of the guest starts 
running way too fast.

Re: kvm live migration reliable?

[Anthony Liguori]

Kent Tong wrote:
> Hi,
>
> I was wondering the kvm live migration function is reliable? I read that if
> the TSC is not monotonically increasing, then the guest will hang. How to
> prevent this?
>   

KVM transfers the tsc as part of the CPU state so it will continue to 
increase monotonically.

However, if you're migrating between two CPUs of different frequency, 
the result will be that the TSC frequency will change.  This could have 
negative effects depends on how much the guest relies on the TSC.  The 
worst case would be if your guest was Linux and was using TSC as a time 
source.  You would likely see severe time drift.

A PV clock source (like KVM_CLOCK) helps this quite a bit.

Regards,

Anthony Liguori

> Thanks!
>
>
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