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

[Don Koch]

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

TIA,
-d