On Wed, 19 Jun 2019 11:04:15 +0200 Sylvain Bauza <sbauza@redhat.com> wrote:
On Wed, Jun 19, 2019 at 12:27 AM Alex Williamson <alex.williamson@redhat.com> wrote:
On Tue, 18 Jun 2019 14:48:11 +0200 Sylvain Bauza <sbauza@redhat.com> wrote:
On Tue, Jun 18, 2019 at 1:01 PM Cornelia Huck <cohuck@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 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?
Well, IMHO, mdevs created by mdevctl could all be persisted or transient just by adding an option when calling mdevctl, like : "mdevctl create-mdev [--transient] <uuid> <pci_id> <type>" where default would be persisting the mdev.
This sounds useful; the caller can avoid fiddling with sysfs entries directly, while not committing to a permanent configuration.
For mdevs *not* created by mdevctl, then a usecase could be "I'd like to ask mdevctl to manage mdevs I already created" and if so, a mdevctl command like : "mdevctl manage-mdev [--transient] <mdev_uuid>"
What kind of 'managing' would this actually enable? If we rely on mdevctl working with sysfs directly for transient devices, I can't really think of anything...
Of course, that would mean that when you list mdevs by "mdev list-all" you wouldn't get mdevs managed by mdevctl. Thoughts ?
- 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.
Well, like I said, I think it's maybe another user case : just using libvirt when you want a guest having vGPUs and then libvirt would create mdevs (so users wouln't need to know at that). That said, for the moment, I think we don't really need it so maybe a new RFE once we at least have mdevctl packaged and supported by RHEL ?
If we allow config file handling directly, libvirt could start using it even without mdevctl present? (Not sure if that makes sense.)
- 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
Yeah, mdevctld--.
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.
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.
Oh yes, please don't premuse mdevctl is only needed by libvirt. FWIW, once mdevctl is supported by RHEL, I'd love to use it for OpenStack Nova at least because I want to persist the mdevs. At the moment, Nova just creates mdevs directly by sysfs and look the existing onces up by sysfs, but upstream community in Nova thinks the mission statement is not about managing mdevs so we don't really want to add in Nova some kind of DB persistence just for mdevs.
So, Nova would basically poke mdevctl, but not interact with the config files directly? Or am I misunderstanding?
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,
Heh, in OpenStack discussing about a file format is possibly one of the longest arguments we already have, so I leave others to say their own opinions but FWIW, as we use Python we tend to prefer YAML files. I don't care about the format tho, just take the most convenient for libvirt I'd say.
Aww, and here I was looking forward to a nice file format discussion... More seriously, as I said in my other reply, .ini style would be good, but using JSON probably gives us more flexibility in the long run.