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Re: [BUG] Migration hv_time rollback

From: Dr. David Alan Gilbert
Subject: Re: [BUG] Migration hv_time rollback
Date: Wed, 16 Sep 2020 12:29:56 +0100
User-agent: Mutt/1.14.6 (2020-07-11)

cc'ing in Vitaly who knows about the hv stuff.

* Antoine Damhet (antoine.damhet@blade-group.com) wrote:
> Hi,
> We are experiencing timestamp rollbacks during live-migration of
> Windows 10 guests with the following qemu configuration (linux 5.4.46
> and qemu master):
> ```
> $ qemu-system-x86_64 -enable-kvm -cpu host,kvm=off,hv_time [...]
> ```

How big a jump are you seeing, and how did you notice it in the guest?


> I have tracked the bug to the fact that `kvmclock` is not exposed and
> disabled from qemu PoV but is in fact used by `hv-time` (in KVM).
> I think we should enable the `kvmclock` (qemu device) if `hv-time` is
> present and add Hyper-V support for the `kvmclock_current_nsec`
> function.
> I'm asking for advice because I am unsure this is the _right_ approach
> and how to keep migration compatibility between qemu versions.
> Thank you all,
> -- 
> Antoine 'xdbob' Damhet

Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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