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

[Daniel P. Berrangé]

On Wed, Jun 19, 2019 at 12:46:33PM -0600, Alex Williamson wrote:
> On Wed, 19 Jun 2019 11:46:59 +0200
> Cornelia Huck <cohuck at redhat.com> wrote:
> 
> > On Wed, 19 Jun 2019 08:28:02 +0100
> > Daniel P. Berrangé <berrange at redhat.com> 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 at redhat.com> wrote:
> > > >     
> > > > > On Tue, Jun 18, 2019 at 1:01 PM Cornelia Huck <cohuck at redhat.com> 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.

NB, for terminology  when libvirt calls something "persistent" it just
means that there's a configuration file recorded on disk, thus when you
stop the thing, you can still query its config & restart it from that
same config later. 

The best solution for libvirt would be to cope with all 4 of those
classes. 1b is the least important for us, so not the end of the
world if it was missing.

> > > 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.


Regards,
Daniel
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