On 7/27/21 4:09 PM, Jonathon Jongsma wrote:
On Tue, Jul 27, 2021 at 3:02 AM Michal Prívozník <mprivozn@xxxxxxxxxx> wrote:
On 7/27/21 12:08 AM, Jonathon Jongsma wrote:
On Mon, Jul 26, 2021 at 9:47 AM Michal Prívozník <mprivozn@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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 am questioning how a user is supposed to correlate the nodedev
representation with mdev_{uuid}_{parent} to a domain using the mdev like
<devices>
<hostdev mode='subsystem' type='mdev' model='vfio-ccw'>
<source>
<address uuid='{uuid}'/>
</source>
</hostdev>
</devices>
Reading uuid it tells me its universal unique
(https://en.wikipedia.org/wiki/Universally_unique_identifier) but now
this is not true for definitions which breaks the term universal in my
small universe since for the defined state of an mdev it becomes
"parental" unique...
What is going to happen to the mdev nodedev object that is just defined
and being started? As this mdev nodedev object at that point in time
gains actually universal uniqueness. Is libvirt supposed to rename the
nodedev object into with name pattern mdev_{uuid}?
Another question is about the autostart feature mdevctl provides for
mdev definitions. Let's assume more than one mdev with the same uuid is
flagged to autostart. What happens is first come first starts all other
fail and a autostart enabled domain using the uuid gets whatever
definition has won the race at system boot?
Isn't the example of individual devices having equivalent characteristic
which Alex used in libvirt represented as pool?
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
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Mit freundlichen Grüßen/Kind regards
Boris Fiuczynski
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