[libvirt PATCH] nodedev: Handle inactive mdevs with the same UUID

[Jonathon Jongsma]

On Thu, Jul 8, 2021 at 3:47 PM Boris Fiuczynski <fiuczy at linux.ibm.com> wrote:
>
> On 7/6/21 9:05 PM, Jonathon Jongsma wrote:
> > Unfortunately, mdevctl supports defining more than one mdev with the
> > same UUID as long as they have different parent devices. So we can't use
> > the UUID alone as a way to uniquely identify these node devices. Append
> > the parent name to ensure uniqueness. For example:
> >
> >      Before: mdev_88a6b868_46bd_4015_8e5b_26107f82da38
> >      After:  mdev_88a6b868_46bd_4015_8e5b_26107f82da38_0000_00_02_0
> >
> > Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1979440
> >
> > Signed-off-by: Jonathon Jongsma <jjongsma at redhat.com>
> > ---
> > QUESTION: Is there any expectation of stability in these device names? I'm not
> > sure to what extent these changes might affect users. Given that support for
> > persistent mdevs is fairly new, there are likely not many users yet.
>
> People using mdevs with libvirt will certainly be impacted by this but I
> am not sure how many there are.
>
> I am wondering if the scenario is even useful since I just tried it out
> and starting the second mdev with the uuid fails.
>
> # mdevctl list -d
> e60cef97-3f6b-485e-ac46-0520f9f66ac2 0.0.0033 vfio_ccw-io manual (active)
> e60cef97-3f6b-485e-ac46-0520f9f66ac2 0.0.0034 vfio_ccw-io manual
> # mdevctl start -u e60cef97-3f6b-485e-ac46-0520f9f66ac2 -p 0.0.0034
> Device exists under different parent
>

Yes, it is expected that you can only have a single device with the
same UUID active at one time. I asked Alex about why mdevctl supported
this feature, and he said he thought it was so that a single VM xml
could run with different mdevs depending on which one was
instantiated. I'm not sure that anybody is actually using this
"feature", but libvirt still has to deal with the possibility that we
could get such a config from mdevctl.

Jonathon