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Re: [Qemu-devel] [libvirt] [PATCH v6 2/2] vl: fix use of --daemonize wit

From: Peter Krempa
Subject: Re: [Qemu-devel] [libvirt] [PATCH v6 2/2] vl: fix use of --daemonize with --preconfig
Date: Tue, 12 Jun 2018 15:10:46 +0200
User-agent: Mutt/1.9.3 (2018-01-21)

On Tue, Jun 12, 2018 at 15:04:42 +0200, Michal Privoznik wrote:
> On 06/12/2018 02:42 PM, Igor Mammedov wrote:
> >>>
> >>> Do we really need to make -daemonize and -preconfig work
> >>> together?  libvirt uses -daemonize only for its initial
> >>> capability probing, which shouldn't require -preconfig at all.
> >>>
> >>> Even on that case, I wonder why libvirt doesn't simply create a
> >>> server socket and waits for QEMU to connect instead of using
> >>> -daemonize as a sync point.
> >>>   
> >>
> >> because libvirt views qemu as well behaved daemon. Should anything go
> >> wrong libvirt reads qemu's stderr and reports error to upper layers.
> > We can keep daemonizing flow in QEMU as it's now.
> > But Eduardo's idea about libvirt created socked + letting QEMU connect to it
> > has a merit. It should fix current deadlock issue with as monitor
> > won't be depending on lead exit event.
> Not sure about the benefits. Currently, libvirt spawns qemu, waits for
> monitor to show up (currently, the timeout dynamic depending on some
> black magic involving guest RAM size) and if it does not show up in time
> it kills qemu. The same algorithm must be kept in place even for case
> when libvirt would pass pre-opened socket to qemu in case qemu deadlocks
> before being able to communicate over qmp. The only advantage I see is
> libvirt would not need to label the socket (set uid:gid, selinux, ...).
> On the other hand, since it would be libvirt creating the socket what
> would happen on libvirtd restart?

Well, if qemu deadlocks just after spewing out the monitor greeting you
end up in the same situation as the timeout code is not applied later
for regular monitor communication.

Depending on how early the preconfig state happens, keeping in the
timeout may be pointless.

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