Re: [libvirt] [RFC PATCH] hostdev: add support for "managed='detach'"

Suggested by Alex Williamson. If you plan to assign a GPU to a virtual machine, but that GPU happens to be the host system console, you likely want it to start out using the host driver (so that boot messages/etc will be displayed), then later have the host driver replaced with vfio-pci for assignment to the virtual machine. However, in at least some cases (e.g. Intel i915) once the device has been detached from the host driver and attached to vfio-pci, attempts to reattach to the host driver only lead to "grief" (ask Alex for details). This means that simply using "managed='yes'" in libvirt won't work. And if you set "managed='no'" in libvirt then either you have to manually run virsh nodedev-detach prior to the first start of the guest, or you have to have a management application intelligent enough to know that it should detach from the host driver, but never reattach to it. This patch makes it simple/automatic to deal with such a case - it adds a third "managed" mode for assigned PCI devices, called "detach". It will detach ("unbind" in driver parlance) the device from the host driver prior to assigning it to the guest, but when the guest is finished with the device, will leave it bound to vfio-pci. This allows re-using the device for another guest, without requiring initial out-of-band intervention to unbind the host driver.