Re: [Qemu-devel] [PATCH 0/3] Add dbus-vmstate

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

On Mon, Jul 8, 2019 at 8:04 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> > The D-Bus protocol can be made to work peer-to-peer, but the most
> > common and practical way is through a bus daemon. This also has the
> > advantage of increased debuggability (you can eavesdrop on the bus and
> > introspect it).
>
> The downside of using the bus daemon is that we have to spawn a new
> instance of dbus-daemon for every QEMU VM that's running on the host,
> which is yet more memory overhead for each VM & another process to
> manage, and yet another thing to go wrong.

dbus-daemon (or dbus-broker) has been optimized to fit on many devices
and use cases, it doesn't take much memory (3mb for my session dbus
right now).

More processes to manage is inevitable. In a near future, we may have
5-10 processes running around qemu. I think dbus-daemon will be one of
the easiest to deal with. (as can be seen in the dbus-vmstate test, it
is very simple to start a private dbus-daemon)

>
> QEMU already has a direct UNIX socket connection to the helper
> processes in question. I'd much rather we just had another direct
> UNIX socket  connection to that helper, using D-Bus peer-to-peer.
> The benefit of debugging doesn't feel compelling enough to justify
> running an extra daemon for each VM.

I wouldn't minor the need for easier debugging. Debugging multiple
processes talking to each other is really hard. Having a bus is
awesome (if not required) in this case.

There are other advantages of using a bus, those come to my mind:

- less connections (bus topology)
- configuring/enforcing policies & limits
- on-demand service activation & discoverability

I also think D-Bus is the IPC of choice for multi-process. It's easier
to use than many other IPC due to the various tools and language
bindings available. Having a common bus is a good incentive to use a
common IPC, instead of a dozen of half-baked protocols.

Nevertheless, I also think we could use D-Bus in peer-to-peer mode,
and I did some investigation. The slirp-helper supports it. We could
teach dbus-vmstate to eastablish peer-to-peer connections. Instead of
receiving a bus address and list of Ids, it could have a list of dbus
peer socket path. Both approaches are not incompatible, but I think
the bus benefits outweigh the downside of running an extra process.