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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Wed, 16 Dec 2015 18:54:05 +0100
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Subject: Re: [Qemu-devel] [PATCH v1] kvm/x86: Remove Hyper-V SynIC timer
	stopping
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To: asmetanin@virtuozzo.com, kvm@vger.kernel.org
Cc: Gleb Natapov <gleb@kernel.org>, "Denis V. Lunev" <den@openvz.org>, Roman Kagan <rkagan@virtuozzo.com>, qemu-devel@nongnu.org



On 14/12/2015 18:01, Andrey Smetanin wrote:
> host                    guest
>                     start periodic stimer
> start periodic timer
> timer expires after 15ms
> send expiration message into guest
> restart periodic timer
>                     ....doing something....
> timer expires again after 15 ms
> msg slot is still not cleared so
> setup ->msg_pending
> restart periodic timer
>                     ....doing something....
>                     process timer msg and clear slot
>                     so ->msg_pending was set:
>                         send EOM into host
> received EOM
> queued call of kvm_hv_process_stimers()
> by KVM_REQ_HV_STIMER
> 
> kvm_hv_process_stimers():
>     ...
>     stimer_stop()
>     if (time_now >= stimer->exp_time)
>             stimer_expiration(stimer);
> But time_now  < stimer->exp_time, so stimer_expiration is not called
> in this case and timer is not restarted. so guest lose timer.

Great, this explains it.

Paolo