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

[Jonathon Jongsma]

On Thu, Jul 8, 2021 at 4:28 PM Jonathon Jongsma <jjongsma at redhat.com> wrote:
>
> 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

By the way, I'm self-NACKing this patch. I have a more comprehensive
patch series coming soon.

Jonathon