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

Don Koch aardvark at krl.com
Wed Apr 8 13:08:14 UTC 2020

On Wed, 8 Apr 2020 10:34:08 +0300
nshirokovskiy wrote:

> 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.
> Nikolay
There was also a startup/resume event. Maybe add a reboot event at that
point? I don't think anyone really cares about when the shutdown occurs
but rather know about the resume.

A side question is: is there a problem with doing a migration if, say,
the reboot was done internally (i.e. someone issued a "reboot" command
from within the VM)?


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