Hey folks, We had some discussions at KVM Forum around mdev live migration and what that might mean for libvirt handling of mdev devices and potential libvirt/mdevctl[1] flows. I believe the current situation is that libvirt knows nothing about an mdev beyond the UUID in the XML. It expects the mdev to exist on the system prior to starting the VM. The intention is for mdevctl to step in here by providing persistence for mdev devices such that these pre-defined mdevs are potentially not just ephemeral, for example, we can tag specific mdevs for automatic startup on each boot. It seems the next step in this journey is to figure out if libvirt can interact with mdevctl to "manage" a device. I believe we've avoided defining managed='yes' behavior for mdev hostdevs up to this point because creating an mdev device involves policy decisions. For example, which parent device hosts the mdev, are there optimal NUMA considerations, are there performance versus power considerations, what is the nature of the mdev, etc. mdevctl doesn't necessarily want to make placement decisions either, but it does understand how to create and remove an mdev, what it's type is, associate it to a fixed parent, apply attributes, etc. So would it be reasonable that for a manage='yes' mdev hostdev device, libvirt might attempt to use mdevctl to start an mdev by UUID and stop it when the VM is shutdown? This assumes the mdev referenced by the UUID is already defined and known to mdevct. I'd expect semantics much like managed='yes' around vfio-pci binding, ex. start/stop if it doesn't exist, leave it alone if it already exists. If that much seems reasonable, and someone is willing to invest some development time to support it, what are then the next steps to enable migration? AIUI, libvirt blindly assumes hostdev devices cannot be migrated. This may already be getting some work due to Jens' network failover support where the attached hostdev doesn't really migrate, but it allows the migration to proceed in a partially detached state so that it can jump back into action should the migration fail. Long term we expect that not only some mdev hostdevs might be migratable, but possibly some regular vfio-pci hostdevs as well. I think libvirt will need to remove any assumptions around hostdev migration and rather rely on introspection of the QEMU process to determine if any devices hold migration blockers (or simply try the migration and let QEMU fail quickly if there are blockers). So assuming we now have a VM with a managed='yes' mdev hostdev device, what do we need to do to reproduce that device at the migration target? mdevctl can dump a device in a json format, where libvirt could use this to define and start an equivalent device on the migration target (potentially this json is extended by mdevctl to include the migration compatibility vendor string). Part of our discussion at the Forum was around the extent to which libvirt would want to consider this json opaque. For instance, while libvirt doesn't currently support localhost migration, libvirt might want to use an alternate UUID for the mdev device on the migration target so as not to introduce additional barriers to such migrations. Potentially mdevctl could accept the json from the source system as a template and allow parameters such as UUID to be overwritten by commandline options. This might allow libvirt to consider the json as opaque. An issue here though is that the json will also include the parent device, which we obviously cannot assume is the same (particularly the bus address) on the migration target. We can allow commandline overrides for the parent just as we do above for the UUID when defining the mdev device from json, but it's an open issue who is going to be smart enough (perhaps dumb enough) to claim this responsibility. It would be interesting to understand how libvirt handles other host specific information during migration, for instance if node or processor affinities are part of the VM XML, how is that translated to the target? I could imagine that we could introduce a simple "first available" placement in mdevctl, but maybe there should minimally be a node allocation preference with optional enforcement (similar to numactl), or maybe something above libvirt needs to take this responsibility to prepare the target before we get ourselves into trouble. Anyway, I hope this captures some of what was discussed at KVM Forum and that we can continue that discussion here to map out the design and tasks to enable vfio/mdev hostdev migration in libvirt. Thanks, Alex [1] https://github.com/mdevctl/mdevctl