7:48 a.m.
On Tue, Jun 18, 2019 at 1:01 PM Cornelia Huck <cohuck(a)redhat.com> wrote:
On Mon, 17 Jun 2019 11:05:17 -0600
Alex Williamson <alex.williamson(a)redhat.com> wrote:
> On Mon, 17 Jun 2019 16:10:30 +0100
> Daniel P. Berrangé <berrange(a)redhat.com> wrote:
>
> > On Mon, Jun 17, 2019 at 08:54:38AM -0600, Alex Williamson wrote:
> > > On Mon, 17 Jun 2019 15:00:00 +0100
> > > Daniel P. Berrangé <berrange(a)redhat.com> wrote:
> > >
> > > > On Thu, May 23, 2019 at 05:20:01PM -0600, Alex Williamson wrote:
> > > > > Hi,
> > > > >
> > > > > Currently mediated device management, much like SR-IOV VF
management,
> > > > > is largely left as an exercise for the user. This is an
attempt
to
> > > > > provide something and see where it goes. I doubt we'll
solve
> > > > > everyone's needs on the first pass, but maybe we'll
solve enough
and
> > > > > provide helpers for the rest. Without further ado, I'll
point
to what
> > > > > I have so far:
> > > > >
> > > > > https://github.com/awilliam/mdevctl
> > > > >
> > > > > This is inspired by driverctl, which is also a bash utility.
mdevctl
> > > > > uses udev and systemd to record and recreate mdev devices for
> > > > > persistence and provides a command line utility for querying,
listing,
> > > > > starting, stopping, adding, and removing mdev devices.
Currently, for
> > > > > better or worse, it considers anything created to be
persistent. I can
> > > > > imagine a global configuration option that might disable this
and
> > > > > perhaps an autostart flag per mdev device, such that mdevctl
might
> > > > > simply "know" about some mdevs but not attempt to
create them
> > > > > automatically. Clearly command line usage help, man pages, and
> > > > > packaging are lacking as well, release early, release often,
plus this
> > > > > is a discussion starter to see if perhaps this is sufficient to
meet
> > > > > some needs.
> > > >
> > > > I think from libvirt's POV, we would *not* want devices to be
made
> > > > unconditionally persistent. We usually wish to expose a choice to
> > > > applications whether to have resources be transient or persistent.
> > > >
> > > > So from that POV, a global config option to turn off persistence
> > > > is not workable either. We would want control per-device, with
> > > > autostart control per device too.
> > >
> > > The code has progressed somewhat in the past 3+ weeks, we still
persist
> > > all devices, but the start-up mode can be selected per device or
with a
> > > global default mode. Devices configured with 'auto' start-up
> > > automatically while 'manual' devices are simply known and
available
to
> > > be started. I imagine we could add a 'transient' mode where we
purge
> > > the information about the device when it is removed or the next time
> > > the parent device is added.
> >
> > Having a pesistent config written out & then purged later is still
> > problematic. If the host crashes, nothing will purge the config file,
> > so it will become a persistent device. Also when listing devices we
> > want to be able to report whether it is persistent or transient. The
> > obvious way todo that is to simply look if a config file exists or
> > not.
>
> I was thinking that the config file would identify the device as
> transient, therefore if the system crashed we'd have the opportunity to
> purge those entries on the next boot as we're processing the entries
> for that parent device. Clearly it has yet to be implemented, but I
> expect there are some advantages to tracking devices via a transient
> config entry or else we're constantly re-discovering foreign mdevs.
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.
- 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.
- 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.)
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/.
-Sylvain
A well-defined config file format is probably a win, even if it only
ends up being used by mdevctl itself.