Re: [PATCH 08/13] vfio: Block device access via device fd until device is opened

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jan 18, 2023 at 09:35:33AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > 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