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Re: Question about migration of a VM between hosts having slight differe

From: Lukas Straub
Subject: Re: Question about migration of a VM between hosts having slight difference in TSC Frequency
Date: Thu, 17 Dec 2020 17:17:24 +0100

On Thu, 17 Dec 2020 14:04:03 +0200
Veselina Radeva <vessy@st6.io> wrote:

> Hi,
> I'm trying to migrate a VM from one host to another where there is a 1KHz
> difference in the TSC frequency between hosts. This results in errors
> caused by this warning
> <https://github.com/qemu/qemu/blob/b785d25e91718a660546a6550f64b3c543af7754/target/i386/kvm.c#L777>,
> so migration fails. On the other hand, I can see few lines before that
> <https://github.com/qemu/qemu/blob/b785d25e91718a660546a6550f64b3c543af7754/target/i386/kvm.c#L761>
> that
> there is a try to allow TSC frequency difference between host and VM if it
> falls within the specified bounds. It seems like `KVM_SET_TSC_KHZ`succeeds
> only when TSC Scaling is available
> <https://github.com/qemu/qemu/blob/2c6605389c1f76973d92b69b85d40d94b8f1092c/linux-headers/linux/kvm.h#L1361>.
> Practically this means that migration between hosts having difference in
> TSC frequency, even when it falls within bounds, is possible only when TSC
> scaling is available.
> On the other hand, in libvirt,
> <https://github.com/libvirt/libvirt/commit/d8e5b4560006590668d4669f54a46b08ec14c1a2>
> migration
> is allowed if TSC frequency difference falls within bounds, no matter if
> TSC scaling is enabled or not.
> So, I was wondering is it intended not to allow migration between hosts
> having differences in TSC frequency if TSC scaling is not available? Or is
> it a bug?
> Thanks in advance,
> Veselina Radeva
> https://st6.io/

AFAIK, this is just a warning. I get it too and migration works fine here. How 
exactly is migration failing for you?

Lukas Straub


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