[libvirt PATCH 6/7] nodedev: Handle inactive mdevs with the same UUID

[Jonathon Jongsma]

On Tue, Jul 27, 2021 at 3:02 AM Michal Prívozník <mprivozn at redhat.com> wrote:
>
> On 7/27/21 12:08 AM, Jonathon Jongsma wrote:
> > On Mon, Jul 26, 2021 at 9:47 AM Michal Prívozník <mprivozn at redhat.com> wrote:
> >>
> >> On 7/23/21 6:40 PM, Jonathon Jongsma wrote:
> >>> Unfortunately, mdevctl supports defining more than one mdev with the
> >>> same UUID as long as they have different parent devices. (Only one of
> >>> these devices can be active at any given time).
> >>>
> >>> This means that we can't use the UUID alone as a way to uniquely
> >>> identify mdev node devices. Append the parent address to ensure
> >>> uniqueness. For example:
> >>>
> >>>     Before: mdev_88a6b868_46bd_4015_8e5b_26107f82da38
> >>>     After:  mdev_88a6b868_46bd_4015_8e5b_26107f82da38_0000_00_02_0
> >>>
> >>> Related: https://bugzilla.redhat.com/show_bug.cgi?id=1979440
> >>>
> >>> Signed-off-by: Jonathon Jongsma <jjongsma at redhat.com>
> >>> ---
> >>>  src/node_device/node_device_driver.c                   | 3 ++-
> >>>  src/node_device/node_device_udev.c                     | 2 +-
> >>>  tests/nodedevmdevctldata/mdevctl-list-multiple.out.xml | 8 ++++----
> >>>  3 files changed, 7 insertions(+), 6 deletions(-)
> >>
> >> The patch looks good, but for some reason it leaves API breakage
> >> aftertaste. But I guess we don't document anywhere what MDEV <name/>
> >> looks like, do we? IOW - we are free to change it if needed.
> >
> >
> > That is true -- it does have a bit of a bad aftertaste. As far as I
> > know, we haven't documented or promised any specific naming format for
> > mdevs. But it could certainly cause some pain for people that are
> > using mdevs already, which might be reason enough to reject or adjust
> > this patch. If a person already has assigned a mdev device to their vm
> > with the old name, the name would change when upgrading libvirt. That
> > could make the domain definition become invalid.
> >
> > But, we have to deal with this situation somehow. We don't have a
> > choice but to handle the case where mdevctl might return multiple
> > defined devices with the same UUID. I considered a couple of other
> > approaches to handling this that I rejected, such as
> > - only add a suffix to the second such device with the same UUID
> >     - rejected because the name of a given device depends on the order
> > in which it was observed and the presence of other devices. For
> > example, if we have two devices: mdev_$UUID and mdev_$UUID_$PARENT,
> > and the first device was undefined, then when libvirt was restarted,
> > mdev_$UUID_$PARENT would change its name to mdev_$UUID because it is
> > now the only device with that UUID
>
> Agreed, this would not make situation any better, in fact worse.
>
> > - name all *active* mdevs mdev_$UUID and use a different scheme for
> > inactive, defined mdevs
> >    - rejected because the device would change names when it changed
> > from inactive to active.
>
> Yeah, this is equally horrible.
>
> > - ignore this situation because probably almost nobody uses multiple
> > devices with the same UUID
> >    - but do they?
>
> Frankly, I don't have enough experience with MDEVs, but since it's
> possible to define an MDEV with an existing UUID, it is possible to
> define it also for the same parent? I mean, if I'd have an MDEV capable
> graphics card and I'd define two MDEVs with the same UUID they would
> both have the same parent, wouldn't they? Because in that case,
> appending PCI address to libvirt's name is not enough.


Nope, it's the opposite in fact. mdevctl allows you to define two or
more mdevs with the same UUID, but only if they have different
parents. Only a single mdev with a given UUID can be activated at the
same time. So among *active* devices, the UUID is indeed unique. But
not among persistent device definitions. For defined devices, the UUID
is guaranteed unique per parent.

I discussed this with Alex a while ago and he thought that the reason
that mdevctl supported this was to allow people to assign an mdev with
a specific UUID to a vm, but that UUID could represent one of several
(equivalent?) devices, depending on which parent device was present in
the host or which device was instantiated. That feature doesn't seem
very compelling to me, and Alex wasn't sure that anybody was actually
using it that way. But the fact remains that mdevctl does allow people
to configure things this way and it seems like we need to be prepared
to deal with it.

I briefly considered somehow rejecting mdevctl configurations in
libvirt that contain duplicate UUIDS, but couldn't figure out a good
way to handle that.

Jonathon