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Re: device compatibility interface for live migration with assigned devices

On 2020/8/18 下午5:36, Cornelia Huck wrote:
On Tue, 18 Aug 2020 10:16:28 +0100
Daniel P. Berrangé <berrange redhat com> wrote:

On Tue, Aug 18, 2020 at 05:01:51PM +0800, Jason Wang wrote:
    On 2020/8/18 下午4:55, Daniel P. Berrangé wrote:

  On Tue, Aug 18, 2020 at 11:24:30AM +0800, Jason Wang wrote:

  On 2020/8/14 下午1:16, Yan Zhao wrote:

  On Thu, Aug 13, 2020 at 12:24:50PM +0800, Jason Wang wrote:

  On 2020/8/10 下午3:46, Yan Zhao wrote:
  we actually can also retrieve the same information through sysfs, .e.g

  |- [path to device]
     |--- migration
     |     |--- self
     |     |   |---device_api
     |    |   |---mdev_type
     |    |   |---software_version
     |    |   |---device_id
     |    |   |---aggregator
     |     |--- compatible
     |     |   |---device_api
     |    |   |---mdev_type
     |    |   |---software_version
     |    |   |---device_id
     |    |   |---aggregator

  Yes but:

  - You need one file per attribute (one syscall for one attribute)
  - Attribute is coupled with kobject
Is that really that bad? You have the device with an embedded kobject
anyway, and you can just put things into an attribute group?

Yes, but all of this could be done via devlink(netlink) as well with low overhead.

[Also, I think that self/compatible split in the example makes things
needlessly complex. Shouldn't semantic versioning and matching already
cover nearly everything?

That's my question as well. E.g for virtio, versioning may not even work, some of features are negotiated independently:

Source features: A, B, C
Dest features: A, B, C, E

We just need to make sure the dest features is a superset of source then all set.

  I would expect very few cases that are more
complex than that. Maybe the aggregation stuff, but I don't think we
need that self/compatible split for that, either.]

  All of above seems unnecessary.

  Another point, as we discussed in another thread, it's really hard to make
  sure the above API work for all types of devices and frameworks. So having a
  vendor specific API looks much better.

  From the POV of userspace mgmt apps doing device compat checking / migration,
  we certainly do NOT want to use different vendor specific APIs. We want to
  have an API that can be used / controlled in a standard manner across vendors.

    Yes, but it could be hard. E.g vDPA will chose to use devlink (there's a
    long debate on sysfs vs devlink). So if we go with sysfs, at least two
    APIs needs to be supported ...
NB, I was not questioning devlink vs sysfs directly. If devlink is related
to netlink, I can't say I'm enthusiastic as IMKE sysfs is easier to deal
with. I don't know enough about devlink to have much of an opinion though.
The key point was that I don't want the userspace APIs we need to deal with
to be vendor specific.
 From what I've seen of devlink, it seems quite nice; but I understand
why sysfs might be easier to deal with (especially as there's likely
already a lot of code using it.)

I understand that some users would like devlink because it is already
widely used for network drivers (and some others), but I don't think
the majority of devices used with vfio are network (although certainly
a lot of them are.)

Note that though devlink could be popular only in network devices, netlink is widely used by a lot of subsystesm (e.g SCSI).


What I care about is that we have a *standard* userspace API for performing
device compatibility checking / state migration, for use by QEMU/libvirt/
OpenStack, such that we can write code without countless vendor specific
code paths.

If there is vendor specific stuff on the side, that's fine as we can ignore
that, but the core functionality for device compat / migration needs to be
To summarize:
- choose one of sysfs or devlink
- have a common interface, with a standardized way to add
   vendor-specific attributes

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