[libvirt] mdevctl: A shoestring mediated device management and persistence utility

[Alex Williamson]

On Fri, 24 May 2019 12:11:06 +0200
Cornelia Huck <cohuck at redhat.com> wrote:

> On Thu, 23 May 2019 17:20:01 -0600
> Alex Williamson <alex.williamson at redhat.com> wrote:
> 
> > Hi,
> > 
> > Currently mediated device management, much like SR-IOV VF management,
> > is largely left as an exercise for the user.  This is an attempt to
> > provide something and see where it goes.  I doubt we'll solve
> > everyone's needs on the first pass, but maybe we'll solve enough and
> > provide helpers for the rest.  Without further ado, I'll point to what
> > I have so far:
> > 
> > https://github.com/awilliam/mdevctl
> > 
> > This is inspired by driverctl, which is also a bash utility.  mdevctl
> > uses udev and systemd to record and recreate mdev devices for
> > persistence and provides a command line utility for querying, listing,
> > starting, stopping, adding, and removing mdev devices.  Currently, for
> > better or worse, it considers anything created to be persistent.  I can
> > imagine a global configuration option that might disable this and
> > perhaps an autostart flag per mdev device, such that mdevctl might
> > simply "know" about some mdevs but not attempt to create them
> > automatically.  Clearly command line usage help, man pages, and
> > packaging are lacking as well, release early, release often, plus this
> > is a discussion starter to see if perhaps this is sufficient to meet
> > some needs.
> > 
> > Originally I thought about making a utility to manage both mdev and
> > SR-IOV VFs all in one, but it seemed more natural to start here
> > (besides, I couldn't think of a good name for the combined utility).
> > If this seems useful, maybe I'll start on a vfctl for SR-IOV and we'll
> > see whether they have enough synergy to become one.
> > 
> > It would be really useful if s390 folks could help me understand
> > whether it's possible to glean all the information necessary to
> > recreate a ccw or ap mdev device from sysfs.  I expect the file where
> > we currently only store the mdev_type to evolve into something that
> > includes more information to facilitate more complicated devices.  For
> > now I make no claims to maintaining compatibility of recorded mdev
> > devices, it will absolutely change, but I didn't want to get bogged
> > down in making sure I don't accidentally source a root kit hidden in an
> > mdev config file.  
> 
> I played a bit with it on my LPAR, and it is at least not obviously
> broken with vfio-ccw :) I don't have any ap devices to play with,
> though.

Awesome, that's a good start.  Thanks for testing!


> > I'm also curious how or if libvirt or openstack might use this.  If
> > nothing else, it makes libvirt hook scripts easier to write, especially
> > if we add an option not to autostart mdevs, or if users don't mind
> > persistent mdevs, maybe there's nothing more to do.
> > 
> > BTW, feel free to clean up by bash, I'm a brute force and ignorance
> > shell coder ;)    
> 
> Not that I'm a good shell coder, but I sent you a pull req at least
> adding a basic help text ;)
> 
> I have not yet looked at most of the code, though.

Pulled, thanks again!  I'll continue to try to fill in some of the
gaps.  I also forgot to mention that the integration with systemd means
that mdevs that mdevctl knows about can be started and stopped with
systemctl, ie:

# systemctl {start|stop} mdev@$UUID.service

You'll see that sbin/mdevctl start/stop-mdev is effectively just an
alias to this.  Thanks,

Alex