Re: device compatibility interface for live migration with assigned devices

* Alex Williamson (alex.williamson(a)redhat.com) wrote: > On Wed, 15 Jul 2020 16:20:41 +0800 > Yan Zhao <yan.y.zhao(a)intel.com&gt; 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(a)redhat.com&gt; wrote: > > > > > > > * Alex Williamson (alex.williamson(a)redhat.com) wrote: > > > > > On Tue, 14 Jul 2020 11:21:29 +0100 > > > > > Daniel P. Berrangé <berrange(a)redhat.com&gt; 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.