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

On Wed, 19 Jun 2019 11:46:59 +0200 Cornelia Huck <cohuck(a)redhat.com&gt; wrote: > On Wed, 19 Jun 2019 08:28:02 +0100 > Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > > > On Tue, Jun 18, 2019 at 04:12:10PM -0600, Alex Williamson wrote: > > > On Tue, 18 Jun 2019 14:48:11 +0200 > > > Sylvain Bauza <sbauza(a)redhat.com&gt; wrote: > > > > > > > On Tue, Jun 18, 2019 at 1:01 PM Cornelia Huck <cohuck(a)redhat.com&gt; wrote: > > > > > > I think we need to reach consensus about the actual scope of the > > > > > mdevctl tool. > > > > > > > > > > > > > > Thanks Cornelia, my thoughts: > > > > > > > > - Is it supposed to be responsible for managing *all* mdev devices in > > > > > the system, or is it more supposed to be a convenience helper for > > > > > users/software wanting to manage mdevs? > > > > > > > > > > > > > The latter. If an operator (or some software) wants to create mdevs by not > > > > using mdevctl (and rather directly calling the sysfs), I think it's OK. > > > > That said, mdevs created by mdevctl would be supported by systemctl, while > > > > the others not but I think it's okay. > > > > > > I agree (sort of), and I'm hearing that we should drop any sort of > > > automatic persistence of mdevs created outside of mdevctl. The problem > > > comes when we try to draw the line between unmanaged and manged > > > devices. For instance, if we have a command to list mdevs it would > > > feel incomplete if it didn't list all mdevs both those managed by > > > mdevctl and those created elsewhere. For managed devices, I expect > > > we'll also have commands that allow the mode of the device to be > > > switched between transient, saved, and persistent. Should a user then > > Hm, what's the difference between 'saved' and 'persistent'? That > 'saved' devices are not necessarily present? It seems like we're coming up with the following classes: 1) transient a) mdevctl created b) foreign 2) defined a) automatic start-up b) manual start-up I was using persistent for 2b), but that's probably not a good name because devices can still be stopped, so they're not really persistently available even in this class.

> > To my mind there shouldn't really need to be a difference between > > transient mdevs created by mdevctrl and mdevs created by an user > > directly using sysfs. Both are mdevs on the running system with > > no config file that you have to enumerate by looking at sysfs. > > This ties back to my belief that we shouldn't need to have any > > config on disk for a transient mdev, just discover them all > > dynamically when required. > > So mdevctl can potentially interact with any mdev device on the system, > it just has to be instructed by a user or software to do so? I think we > can work with that. Some TBDs around systemd/init support for transient devices and how transient devices can be promoted to defined. For instance if a vfio-ap device requires matrix programming after instantiation, can we glean that programming from sysfs or is there metadata irrecoverably lost if no config file is created for a transient device? This would also imply that a 1b) foreign device could not be promoted to 2x) defined device.