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

[Halil Pasic]

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,
> > 

[..]

> > 
> > 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.
> 

Sorry for being late...

I guess for vfio-ccw one needs to make sure that the ccw device is bound
to the vfio-ccw driver first, and only after that can one use  
create-mdev to create the mdev on top of the subchannel.

So to make this work persistently (survive a reboot) one would need to
take care of the subchannel getting bound to the right vfio_ccw driver
before mdevctl is called. Right?

BTW how does this concurrence situation between the drivers io_subchannel
and vfio_ccw work? Especially if both are build in?

> > 
> > 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.
> > 

+1

@Alex: I'm curious what is the big management picture for non-auto looks
like.

Regards,
Halil

[..]