systemd: Tagging /dev/virtio-ports/* for systemd

[Lennart Poettering]

On Tue, 05.07.11 16:54, Daniel P. Berrange (berrange at redhat.com) wrote:

Heya,

> On Tue, Jul 05, 2011 at 04:47:18PM +0100, Richard W.M. Jones wrote:
> > [Is there a Fedora-specific systemd list?  Not that I can find.]
> > 
> > We'd like to request that virtio-serial devices (/dev/virtio-ports/*)
> > are tagged so we can use them as systemd devices.  Dan Berrange thinks
> > the right incantation is to add:
> > 
> > SUBSYSTEM=="virtio-ports", TAG+="systemd"
> > 
> > to /lib/udev/rules.d/99-systemd.rules
> 
> The goal is that we want to automagically run /usr/sbin/guestfsd
> when /dev/virtio-ports/org.libguestfs.channel.0 is present. For
> this I believe we need to have a '.device' unit for the virtio-port
> device populated from the above udev rule, so we can in turn have a
> guestfsd.service unit looking like:

I think adding such a rule to systemd's unit file is not a bad idea, but
since the use case here is very specific to your app, another option
would be to add an app-specific udev rule for this. (See below)

> [Unit]
> Description=libguestfs management daemon
> BindTo=dev-virtio\x2dports-org.libguestfs.channel.0.device
> After=dev-virtio\x2dports-org.libguestfs.channel.0.device
> 
> [Service]
> ExecStart=-/usr/sbin/guestfsd

Prefixing the binary path with "-" will result in the exit code of
guestfsd be ignored, i.e. we wouldn't put the service into failed state
if it crashes (or exits otherwise abnormally). I'd encourage never to
prefix with "-" unless you have a really good reason to.

> Restart=always
> RestartSec=0
> 
> [Install]
> WantedBy=multi-user.target

If you use "WantedBy=multi-user.target" then this unit would be started
at boot, and delayed until the device in question shows up. If it never
shows up (for example because you boot on bare metal), then it would
have to timeout, which wouldn't be particularly pretty.

I wonder if it wouldn't be nicer to use the device showing up as _only_
trigger to spawn the service. This would be nicer I think because it
would spawn the service only if run in a VM. If you run the machine on
bare metal, then it wouldn't be started at all, and would not have to
timeout. (And if I grok this properly, then the virtio serial ports are
even hotpluggable, which makes this even more interesting) To implement
a scheme like that, you'd just ship a udev rules file which you install
to /lib/udev/rules/99-guestfs.rules with contents
like this:

SUBSYSTEM=="virtio-ports", ATTR{name}=="org.libguestfs.channel.0", TAG+="systemd", ENV{SYSTEMD_WANTS}="guestfsd.service"

(untested, I hope the match is right)

It's up to you whether you prefer the "spawn unconditionally but wait
for the device to show up" approach or my suggestion of "spawn if and
when the device shows up".

Either way I have now added to git a patch that marks virtio ports for
exposure in systemd, by marking them with "systemd".

Lennart

-- 
Lennart Poettering - Red Hat, Inc.