qemu no-shutdown feature in libvirt

[Daniel P. Berrangé]

On Tue, Dec 01, 2020 at 07:38:51PM +0100, Jiri Denemark wrote:
> On Tue, Dec 01, 2020 at 17:53:30 +0000, Daniel P. Berrangé wrote:
> > On Tue, Dec 01, 2020 at 05:09:26PM +0100, Jiri Denemark wrote:
> > > On Tue, Dec 01, 2020 at 15:34:05 +0100, Peter Krempa wrote:
> > > > On Mon, Nov 30, 2020 at 14:45:18 -0500, Luna Xu wrote:
> > > > > Hi developers,
> > > > 
> > > > Hi,
> > > > 
> > > > [...]
> > > > 
> > > > > When I start a VM with the qemu command I can specify the -no-shutdown
> > > > > flag so that my qemu process doesn't quit even if I shutdown the VM
> > > > > from the inside (issue shutdown or halt command inside VM). The VM is
> > > > > shutdown but the qemu process is not killed so that the jobs I
> > > > > submitted before for example backup (drive-backup) can continue to run
> > > > > even after VM gets shutdown by the user, and I'm able to check the
> > > > > status of the job through qmp commands (by communicating with the qemu
> > > > > process).
> > > > 
> > > > As I've noted in my reply on libvirt-users, we currently don't support
> > > > this on the shutdown part of the VM lifecycle.
> > > > 
> > > > On the startup part you can use VIR_DOMAIN_START_PAUSED for a VM which
> > > > didn't yet excecute any guest instructions.
> > > > 
> > > > > Is anyone aware of any project that is currently implementing this
> > > > > functionality? Thanks for reading.
> > > > 
> > > > I don't think that there's anybody working on it.
> > > > 
> > > > If you are interested in implementing the feature the idea would be to
> > > > add a 'pause' action for <on_poweroff>pause</on_poweroff> along with a
> > > > new 'virDomainPausedReason' such as VIR_DOMAIN_PAUSED_POWEROFF and
> > > 
> > > > implement the lifecycle transition from PAUSED -> RUNNING via
> > > > qemuDomainResume, which would in this case have to issue a
> > > > 'system-reset' qmp command and emit the appropriate events.
> > > 
> > > This is weird, the domain was paused just before being shut down so
> > > resuming it should logically just resume the paused process and finish
> > > the shutdown. On the other hand, domain shutdown as a result of calling
> > > resume is weird too.
> > > 
> > > So how about just adding a check to qemuDomainResume to report an error
> > > if you try to resume a domain which was paused for shutdown?
> > 
> > Note we have a virDomainReset() API that does a full machine/cpu reset,
> > so if we're in the puased state after shutdown, then a reset followed
> > by a resume is a valid way to boot the guest OS fresh. This is in fact
> > how we fake graceful reboots.
> 
> Sure, I just didn't want virDomainResume to automagically do the reset
> part. I guess you're suggesting that we should not report an error
> because virDomainReset would not change the domain state (or would it?)
> and thus reporting an error would block this scenario. So in that case,
> we should not report any error and in just try to resume the CPUs. In
> other words, this would mean no change is really needed in
> qemuDomainResume.

We should be in the "SHUTDOWN" state when paused at the end of shutdown.
Normally you'd never see this stsate as we immediately go into SHUTOFF.
A virDomainReset should likely transition back to "RUNNING", but "PAUSED".

So we can block on the state in virDomainResume if we're in SHUTDOWN.

Regards,
Daniel
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