Re: [RFC] dbus-vmstate: Connect to the dbus only during the migration phase

[Daniel P . Berrangé]

On Thu, Nov 19, 2020 at 06:28:55PM +0000, Priyankar Jain wrote:
> Today, dbus-vmstate maintains a constant connection to the dbus. This is
> problematic for a number of reasons:
> 1. If dbus-vmstate is attached during power-on, then the device holds
>    the unused connection for a long period of time until migration
>    is triggered, thus unnecessarily occupying dbus.
> 2. Similarly, if the dbus is restarted in the time period between VM
>    power-on (dbus-vmstate initialisation) and migration, then the
>    migration will fail. The only way to recover would be by
>    re-initialising the dbus-vmstate object.
> 3. If dbus is not available during VM power-on, then currently dbus-vmstate
>    initialisation fails, causing power-on to fail.
> 4. For a system with large number of VMs, having multiple QEMUs connected to
>    the same dbus can lead to a DoS for new connections.

The expectation is that there is a *separate* dbus daemon created for
each QEMU instance. There should never be multiple QEMUs connected to
the same dbus instance, nor should it ever connect to the common dbus
instances provided by most Linux distros.

None of these 4 issues should apply when each QEMU has its own dedicated
dbus instance AFAICT.


Regards,
Daniel
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