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

["Radim Krčmář" <rkrcmar@redhat.com>]

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+)?

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).

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

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

Thanks.