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

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

> be allowed to promote an unmanaged device to one of these modes via the > same command? Should they be allowed to stop an unmanaged device > through driverctl? Through systemctl? These all seem like reasonable > things to do, so what then is the difference between transient and > unmanaged mdev and is mdevctl therefore managing all mdevs, not just > those it has created? 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.

> > - Do we want mdevctl to manage config files for individual mdevs, or > > > are they supposed to be in a common format that can also be managed > > > by e.g. libvirt? > > > > > > > Unless I misunderstand, I think mdevctl just helps to create mdevs for > > being used by guests created either by libvirt or QEMU or even others. > > How a guest would allocate a mdev (ie. saying "I'll use this specific mdev > > UUID") is IMHO not something for mdevctl. > > Right, mdevctl isn't concerned with how a specific mdev is used, but I > think what Connie is after is more the proposal from Daniel where > libvirt can essentially manage mdevctl config files itself and then > only invoke mdevctl for the dirty work of creating and deleting > devices. In fact, assuming systemd, libvirt could avoid direct > interaction with mdevctl entirely, instead using systemctl device units > to start and stop the mdevs. Maybe where that proposal takes a turn is > when we again consider non-systemd hosts, where maybe mdevctl needs to > write out an init script per mdev and libvirt injecting itself into > manipulation of the config files would either need to perform the same > or fall back to mdevctl. Unfortunately there seems to be an ultimatum > to either condone external config file manipulation or expand the scope > of the project into becoming a library. Is mdevctl really tackling a problem that is complex enough that we will gain significantly by keeping the config files private and forcing use of a CLI or Library to access them ? The amount of information that we need to store per mdev looks pretty small, with minimal compound structure. To me it feels like we can easily define a standard config format without suffering any serious long term pain, as the chances we'd need to radically change it look minimal.

> > - Should mdevctl be a stand-alone tool, provide library functions, or > > > both? Related: should it keep any internal state that is not written > > > to disk? (I think that also plays into the transient vs. persistent > > > question.) > > I don't think we want an mdevctld, if that's what you mean by internal > state not written to disk. I think we ideally want all state in the > mdev config files or discerned through sysfs. How we handle > non-systemd hosts may throw a wrench in that though since currently the > systemd integration relies on a template to support arbitrary mdevs and > I'm not sure how to replicate that in other init services. If we need > to dynamically manage per mdev init files in addition to config files, > we're not so self contained. The most important part of the init script integration is just the bulk creation of mdevs on startup. I think this could be handled on non-systemd hosts via a fairly dumb init script that does this something approximating this: for dev in `mdevctl list` do mdevctrl get-autostart $dev test $? = 0 && mdevctrl start $dev done ie, iterate over all configs. If the config is marked to autostart, then start it.

> > FWIW, I'd love using mdevctl for OpenStack (Nova) just at least for > > creating persisted mdevs (ie. mdevs that would be recreated after rebooting > > using systemctl). That's the real use case I need. > > Whether libvirt would internally support mdevctl would be nice but that's > > not really something Nova needs, so I leave others providing their own > > thoughts. > > > > > > My personal opinion is that mdevctl should be able to tolerate mdevs > > > being configured by other means, but probably should not try to impose > > > its own configuration if it detects that (unless explicitly asked to do > > > so). Not sure how feasible that goal is. > > > > > > That's what I misunderstand : in order to have a guest using a vGPU, you > > need to do two things : > > 1/ create the mdev > > 2/ allocate this created dev to a specific guest config > > > > Of course, we could imagine a way to have both steps to be done directly by > > libvirt, but from my opinion, mdevctl is really helping 1/ and not 2/.

> > Yep, we also don't want to presume libvirt is the only consumer here. > mdevctl should also support other VM management tools, users who write > their own management scripts, and even non-VM related use cases. > > > > A well-defined config file format is probably a win, even if it only > > > ends up being used by mdevctl itself. > > Yes, regardless of whether others touch them, conversion scripts on > upgrade should be avoided. Do we need something beyond a key=value > file? So far we're only storing the mdev type and startup mode, but > vfio-ap clearly needs more, apparently key=value1,value2,... type > representation. Still, I think I'd prefer simple over jumping to xml > or json or yaml. Thanks, For libvirt our preference would be something we can easily support without having to write new parsers. I'm not going to suggest XML, but JSON is probably our highest preference. If not then a simple flat file with one line of key="value" per setting is something we already parse for /etc/libvirt/libvirtd.conf file. For slightly more structure the .ini style file is also good. That's basically just flat key=value pairs, but with [section] headers so you can represent some level of structured data.