Re: device-specific VFIO drivers

The Linux kernel has recently added support for device-specific VFIO drivers to be used (instead of the current generic, one-size-fits-all vfio_pci driver) when assigning a device to a guest with VFIO. The intent of this is to (for example) support APIs for device-specific setup that needs to be done before the device is handed over to the guest, and to support migration of guests using these devices. More details can be found in the comments of the patch emails here: https://lore.kernel.org/kvm/20210826103912.128972-1-yishaih@nvidia.com/ There are a couple of issues that need to be resolved before that will work well with libvirt-managed guests: 1) Although they've outlined a system for determining the best / most specific driver for a device by extending the existing modules aliases, the kernel people have left it up to userspace to actually parse the module.aliases info to determine the best driver for a device. They've even "thrown us a bone" in the form of an example python script to do just that: https://github.com/maxgurtovoy/linux_tools/blob/main/vfio/bind_vfio_pci_d... Currently, if a device is assigned to a guest with <hostdev managed='yes'> (the default) then libvirt will unbind the device from whatever host driver its bound to, and bind it to the vfio_pci driver (since, up to now, that has been the only driver available). In the future, if we want to be able to take advantage of the extended capabilities provided by the device-specific vfio drivers, we will need to figure out which driver to load. I personally don't like the idea of embedding the logic from the above example program into libvirt - this really seems to me like something that should be implemented in one place for use by many consumers (libvirt being one of the consumers). One suggestion made by Alex Williamson (in a private discussion, not on a mailing list) was that possibly we could get the driverctl utility to add this functionality, and then call driverctl to learn the name of the best-fit vfio driver for a device. I haven't yet looked at it, but I've been told that driverctl is a bash script :-O. If we decide that's a good route, perhaps we could also convince someone to convert drivertctl to rust, similar to what jjongsma did with mdevctl (nudge nudge, hint hint). Or maybe I'm making it into a bigger deal than it needs to be, and we should just implement the logic of the python script up above directly in libvirt. Does anyone have an opinion?

2) There may be cases where, in spite of a device-specific driver being available, the user prefers to use the generic vfio_pci driver. To support that we will need to have a place in our config to set the driver name. We already have a <driver> subelement of <hostdev> that was originally added to allow choosing between VFIO device assignment and legacy KVM device assignment: <driver name='vfio|kvm'/> All support for KVM device assignment was removed a few years ago, so in practice the driver name is always "vfio". The most natural looking way to support device-specific drivers would be to use this name attribute to specify the driver name, e.g. if you wanted to let libvirt select the best driver, you would use: <driver name='vfio'/> (what it's currently set to in everyone's configs). But if you wanted to force use of the generic driver, you'd use: <driver name='vfio_pci'/> or if you wanted to force use of another driver that wasn't the 'best fit' according to the module aliases, you could use, e.g.: <driver name='vfio_pci_xyzzy'/> I'm uncomfortable with the fact that we're effectively "re-using" the name attribute for a new purpose though - up until now it has been "which device assignment method?", but this changes it to "which vfio driver should be loaded?".

So maybe we need a new element. I'm not sure what to call it though. How about this? <driver name='vfio' modname='vfio_pci'/> Does anyone have a better idea for the name?