Re: [PATCH] qga: Start qemu-ga service after NetworkManager start

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Aug 17, 2023 at 12:47:48PM +0200, Denis V. Lunev wrote:
> On 8/17/23 12:26, Daniel P. Berrangé wrote:
> > On Thu, Aug 17, 2023 at 04:04:46AM +0800, Efim Shevrin via wrote:
> > > From: Fima Shevrin <efim.shevrin@virtuozzo.com>
> > > 
> > > When the guest OS starts, qemu-ga sends an event to the host.
> > > This event allows services on the host to start configuring
> > > the already running guest OS. When configuring network settings,
> > > it is possible that an external service will receive a signal
> > > from qemu-ga about the start of guest OS, while NetworkManager
> > > may not be running yet. Therefore, network setting may not
> > > be available. With the current patch, we eliminate the described
> > > race condition between qemu-ga and NetworkManager for guest OS
> > > network setting cases.
> > A host mgmt app should not be assuming that networking is
> > running at all, and if it is, the guest might not even be
> > on the same subnet as the host. Conceivably someone could
> > even be using QGA to inject files to configure networking.
> > 
> > Overall, IMHO, it is wrong to correlate host notification
> > that QGA is running, with any assumption about state of
> > the guest networking.
> > 
> > IOW, I don't think we should make an attempt to serialize
> > startup of QGA after guest networking. The QGA should be
> > run as soon as its data transport (virtio-serial/vsock)
> > is available, which is the current state.
> > 
> > If a host consumer of QGA wants to do something with
> > guest networking, they should check for guest network
> > state with the 'guest-network-get-interfaces' command.
> 
> This is very tight question at my opinion.
> 
> Frankly speaking my original intention here would be
> to put QGA at the very end of the startup sequence
> in order to avoid complications. In the other case
> any host side software, which would like to control
> the guest behavior will be in trouble.
> 
> For example, yet another common application is a
> guest backup, here I would say application based
> backup. In a normal case this is done through
>    /etc/qemu-ga/fsfreeze-hook

It will already be started after local-fs.target, since it does not
have DefaultDependencies=no set. This should be sufficient for
fsfreeze use cases IIUC.

> I would say that agent should start very last, in the
> other case *ANY* software working through the agent
> must take efforts to avoid such transient state, which
> is complicated.



With regards,
Daniel
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