Re: [libvirt] RFC: Creating mediated devices with libvirt

>> So, just for clarification of the concept, the device with ^this UUID will have >> had to be defined by the nodedev API by the time we start to edit the domain >> XML in this manner in which case the only thing the autocreate=yes would do is >> to actually create the mdev according to the nodedev config, right? Continuing >> with that thought, if UUID doesn't refer to any of the inactive configs it will >> be an error I suppose? What about the fact that only one vgpu type can live on >> the GPU? even if you can successfully identify a device using the UUID in this >> way, you'll still face the problem, that other types might be currently >> occupying the GPU and need to be torn down first, will this be automated as >> well in what you suggest? I assume not. > > Technically we shouldn't need the node device to exist at the time we > define the XML - only at the time we start the guest, does the node > device have to exist. eg same way you list a virtual network as the > source of a guest NIC, but that virtual network doesn't have to actually > have been defined & started until the guest starts. > > If there are constraints that a pGPU can only support a certain combination > of vGPUs at any single point in time, doesn't the kernel already enforce > that when you try to create the vGPU in sysfs. IOW, we merely need to try > to create the vGPU, and if the kernel mdev driver doesn't allow you to mix > that with the other vGPUs that already exist, then we'd just report an > error from virNodeDeviceCreate, and that'd get propagated back as the > error for the virDomainCreate call. > >> >>> </source> >>> </hostdev> >>> </devices> >>> >>> In the QEMU driver, then the only change required is >>> >>> if (def->autocreate) >>> virNodeDeviceCreate(dev) >> >> Aha, so if a device gets torn down on shutdown, we won't face the problem with >> some other devices being active, all of them will have to be in the inactive >> state because they got torn down during the last shutdown - that would work. > > I'm not sure what the relationship with other active devices is relevant > here. The virNodeDevicePtr we're accesing here is a single vGPU - if other > running guests have further vGPUs on the same pGPU, that's not really > relevant. Each vGPU is created/deleted as required. I think he's talking about devices that were previously used by other domains that are no longer active. Since they're also automatically destroyed, they're not a problem.