[libvirt] [PATCH v2 1/1] qemu: hide details of fake reboot

nshirokovskiy nshirokovskiy at virtuozzo.com
Wed Apr 8 07:34:08 UTC 2020

On 07.04.2020 20:31, Don Koch wrote:
> On Thu, 19 Dec 2019 13:50:19 +0300
> Nikolay Shirokovskiy wrote:
>> If we use fake reboot then domain goes thru running->shutdown->running
>> state changes with shutdown state only for short period of time.  At
>> least this is implementation details leaking into API. And also there is
>> one real case when this is not convinient. I'm doing a backup with the
>> help of temporary block snapshot (with the help of qemu's API which is
>> used in the newly created libvirt's backup API). If guest is shutdowned
>> I want to continue to backup so I don't kill the process and domain is
>> in shutdown state. Later when backup is finished I want to destroy qemu
>> process. So I check if it is in shutdowned state and destroy it if it
>> is. Now if instead of shutdown domain got fake reboot then I can destroy
>> process in the middle of fake reboot process.
>> After shutdown event we also get stop event and now as domain state is
>> running it will be transitioned to paused state and back to running
>> later. Though this is not critical for the described case I guess it is
>> better not to leak these details to user too. So let's leave domain in
>> running state on stop event if fake reboot is in process.
>> Reconnection code handles this patch without modification. It detects
>> that qemu is not running due to shutdown and then calls qemuProcessShutdownOrReboot
>> which reboots as fake reboot flag is set.
>> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy at virtuozzo.com>
> Question regarding this change: in the on-line documentation for virDomainReboot(),
> it states:
>    Due to implementation limitations in some drivers (the qemu driver,
>    for instance) it is not advised to migrate or save a guest that is
>    rebooting as a result of this API. Migrating such a guest can lead to a
>    plain shutdown on the destination.
> Is this still the case? 

Hi, Don. Yeah this is still the case.

> In any event, how does one know when the reboot has
> finished (assuming the VM reacted to it)?

Unfortunately there is no event for reboot. Before the patch there was
SHUTDOWN event but only when reboot is done thru ACPI. Now when fake
reboot impl is more incapsulated there is no more SHUTDOWN event just as
for reboot from guest.

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