Re: device compatibility interface for live migration with assigned devices

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Alex Williamson (alex.williamson@redhat.com) wrote:
> On Wed, 15 Jul 2020 16:20:41 +0800
> Yan Zhao <yan.y.zhao@intel.com> wrote:
> 
> > On Tue, Jul 14, 2020 at 02:59:48PM -0600, Alex Williamson wrote:
> > > 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,
> > >  
> > Yes, I agree kernel could expose such sysfs interface to educate
> > openstack how to filter out devices. But I still think the proposed
> > migration_version (or rename to migration_compatibility) interface is
> > still required for libvirt to do double check.
> > 
> > In the following scenario: 
> > 1. openstack chooses the target device by reading sysfs interface (of json
> > format) of the source device. And Openstack are now pretty sure the two
> > devices are migration compatible.
> > 2. openstack asks libvirt to create the target VM with the target device
> > and start live migration.
> > 3. libvirt now receives the request. so it now has two choices:
> > (1) create the target VM & target device and start live migration directly
> > (2) double check if the target device is compatible with the source
> > device before doing the remaining tasks.
> > 
> > Because the factors to determine whether two devices are live migration
> > compatible are complicated and may be dynamically changing, (e.g. driver
> > upgrade or configuration changes), and also because libvirt should not
> > totally rely on the input from openstack, I think the cost for libvirt is
> > relatively lower if it chooses to go (2) than (1). At least it has no
> > need to cancel migration and destroy the VM if it knows it earlier.
> > 
> > So, it means the kernel may need to expose two parallel interfaces:
> > (1) with json format, enumerating all possible fields and comparing
> > methods, so as to indicate openstack how to find a matching target device
> > (2) an opaque driver defined string, requiring write and test in target,
> > which is used by libvirt to make sure device compatibility, rather than
> > rely on the input accurateness from openstack or rely on kernel driver
> > implementing the compatibility detection immediately after migration
> > start.
> > 
> > Does it make sense?
> 
> No, libvirt is not responsible for the success or failure of the
> migration, it's the vendor driver's responsibility to encode
> compatibility information early in the migration stream and error
> should the incoming device prove to be incompatible.  It's not
> libvirt's job to second guess the management engine and I would not
> support a duplicate interface only for that purpose.  Thanks,

libvirt does try to enforce it for other things; trying to stop a bad
migration from starting.

Dave

> Alex
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK