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

[Cornelia Huck]

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.

> 
> 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.

> Thanks,
> 
> Alex