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

[Lennart Poettering]

On Tue, 05.07.11 20:36, Lennart Poettering (mzerqung at 0pointer.de) wrote:

> > > 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)

Oops, wanted to write "systemd's udev rules file" here, and not
"systemd's unit file".

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

So, I am a bit confused now after reading this:

http://fedoraproject.org/wiki/Features/VirtioSerial

How does /dev/hvcxxx relate to these virtio ports?

hvc ports are tagged "systemd" anyway in udev, so if this is the same
thing, why do we have to tag the virtio ports too?

Can you explain how hvc and virtio relate to each other and under which
kernel device names they show up in udev and how they correspond?

Lennart

-- 
Lennart Poettering - Red Hat, Inc.