On Wed, Jan 18, 2023 at 09:35:33AM +0000, Tian, Kevin wrote: > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx> > > Sent: Tuesday, January 17, 2023 9:50 PM > > > > Allow the vfio_device file to be in a state where the device FD is > > opened but the device cannot be used by userspace (i.e. its .open_device() > > hasn't been called). This inbetween state is not used when the device > > FD is spawned from the group FD, however when we create the device FD > > directly by opening a cdev it will be opened in the blocked state. > > > > In the blocked state, currently only the bind operation is allowed, > > other device accesses are not allowed. Completing bind will allow user > > to further access the device. > > > > This is implemented by adding a flag in struct vfio_device_file to mark > > the blocked state and using a simple smp_load_acquire() to obtain the > > flag value and serialize all the device setup with the thread accessing > > this device. > > > > Due to this scheme it is not possible to unbind the FD, once it is bound, > > it remains bound until the FD is closed. > > > > My question to the last version was not answered... > > Can you elaborate why it is impossible to unbind? Is it more an > implementation choice or conceptual restriction? At least for the implementation it is due to the use of the lockless test for bind. It can safely handle unbind->bind but it cannot handle bind->unbind. To allows this we'd need to add a lock on all the vfio ioctls which seems costly. Jason
