Re: [libvirt] libvirt mdev migration, mdevctl integration

On Mon, Nov 18, 2019 at 10:06:34AM -0700, Alex Williamson wrote: > 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? The first step is to deal with our virNodeDevice APIs. Currently we have - Listing devices via ( virConnectListAllNodeDevices ) - Create transient device ( virNodeDeviceCreateXML ) - Delete transient device ( virNodeDeviceDestroy ) The create/delete APIs only deal with NPIV HBAs right now, so we need to extend that to deal with mdevs as first step. This entails defining an XML format that can represent the information we need about an mdev.