Re: device compatibility interface for live migration with assigned devices

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 14 Jul 2020 18:19:46 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Alex Williamson (alex.williamson@redhat.com) wrote:
> > On Tue, 14 Jul 2020 11:21:29 +0100
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> >   
> > > On Tue, Jul 14, 2020 at 07:29:57AM +0800, Yan Zhao wrote:  
> > > > hi folks,
> > > > we are defining a device migration compatibility interface that helps upper
> > > > layer stack like openstack/ovirt/libvirt to check if two devices are
> > > > live migration compatible.
> > > > The "devices" here could be MDEVs, physical devices, or hybrid of the two.
> > > > e.g. we could use it to check whether
> > > > - a src MDEV can migrate to a target MDEV,
> > > > - a src VF in SRIOV can migrate to a target VF in SRIOV,
> > > > - a src MDEV can migration to a target VF in SRIOV.
> > > >   (e.g. SIOV/SRIOV backward compatibility case)
> > > > 
> > > > The upper layer stack could use this interface as the last step to check
> > > > if one device is able to migrate to another device before triggering a real
> > > > live migration procedure.
> > > > we are not sure if this interface is of value or help to you. please don't
> > > > hesitate to drop your valuable comments.
> > > > 
> > > > 
> > > > (1) interface definition
> > > > The interface is defined in below way:
> > > > 
> > > >              __    userspace
> > > >               /\              \
> > > >              /                 \write
> > > >             / read              \
> > > >    ________/__________       ___\|/_____________
> > > >   | migration_version |     | migration_version |-->check migration
> > > >   ---------------------     ---------------------   compatibility
> > > >      device A                    device B
> > > > 
> > > > 
> > > > a device attribute named migration_version is defined under each device's
> > > > sysfs node. e.g. (/sys/bus/pci/devices/0000\:00\:02.0/$mdev_UUID/migration_version).
> > > > userspace tools read the migration_version as a string from the source device,
> > > > and write it to the migration_version sysfs attribute in the target device.
> > > > 
> > > > The userspace should treat ANY of below conditions as two devices not compatible:
> > > > - any one of the two devices does not have a migration_version attribute
> > > > - error when reading from migration_version attribute of one device
> > > > - error when writing migration_version string of one device to
> > > >   migration_version attribute of the other device
> > > > 
> > > > The string read from migration_version attribute is defined by device vendor
> > > > driver and is completely opaque to the userspace.
> > > > for a Intel vGPU, string format can be defined like
> > > > "parent device PCI ID" + "version of gvt driver" + "mdev type" + "aggregator count".
> > > > 
> > > > for an NVMe VF connecting to a remote storage. it could be
> > > > "PCI ID" + "driver version" + "configured remote storage URL"
> > > > 
> > > > for a QAT VF, it may be
> > > > "PCI ID" + "driver version" + "supported encryption set".
> > > > 
> > > > (to avoid namespace confliction from each vendor, we may prefix a driver name to
> > > > each migration_version string. e.g. i915-v1-8086-591d-i915-GVTg_V5_8-1)  
> > 
> > It's very strange to define it as opaque and then proceed to describe
> > the contents of that opaque string.  The point is that its contents
> > are defined by the vendor driver to describe the device, driver version,
> > and possibly metadata about the configuration of the device.  One
> > instance of a device might generate a different string from another.
> > The string that a device produces is not necessarily the only string
> > the vendor driver will accept, for example the driver might support
> > backwards compatible migrations.  
> 
> (As I've said in the previous discussion, off one of the patch series)
> 
> My view is it makes sense to have a half-way house on the opaqueness of
> this string; I'd expect to have an ID and version that are human
> readable, maybe a device ID/name that's human interpretable and then a
> bunch of other cruft that maybe device/vendor/version specific.
> 
> I'm thinking that we want to be able to report problems and include the
> string and the user to be able to easily identify the device that was
> complaining and notice a difference in versions, and perhaps also use
> it in compatibility patterns to find compatible hosts; but that does
> get tricky when it's a 'ask the device if it's compatible'.

In the reply I just sent to Dan, I gave this example of what a
"compatibility string" might look like represented as json:

{
  "device_api": "vfio-pci",
  "vendor": "vendor-driver-name",
  "version": {
    "major": 0,
    "minor": 1
  },
  "vfio-pci": { // Based on above device_api
    "vendor": 0x1234, // Values for the exposed device
    "device": 0x5678,
      // Possibly further parameters for a more specific match
  },
  "mdev_attrs": [
    { "attribute0": "VALUE" }
  ]
}

Are you thinking that we might allow the vendor to include a vendor
specific array where we'd simply require that both sides have matching
fields and values?  ie.

  "vendor_fields": [
    { "unknown_field0": "unknown_value0" },
    { "unknown_field1": "unknown_value1" },
  ]

We could certainly make that part of the spec, but I can't really
figure the value of it other than to severely restrict compatibility,
which the vendor could already do via the version.major value.  Maybe
they'd want to put a build timestamp, random uuid, or source sha1 into
such a field to make absolutely certain compatibility is only determined
between identical builds?  Thanks,

Alex