Re: [PATCH v3 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM

[Alex Williamson]

On Thu, 19 May 2022 14:33:11 -0400
Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:

> Rather than relying on a notifier for associating the KVM with
> the group, let's assume that the association has already been
> made prior to device_open.  The first time a device is opened
> associate the group KVM with the device.
> 
> This fixes a user-triggerable oops in GVT.

It seems this has traded an oops for a deadlock, which still exists
today in both GVT-g and vfio-ap.  These are the only vfio drivers that
care about kvm, so they make use of kvm_{get,put}_kvm(), where the
latter is called by their .close_device() callbacks.

.close_device() is called holding the group->group_lock, or at the time
of this commit group->group_rwsem.  The remaining call chain looks like
this:

kvm_put_kvm
 -> kvm_destroy_vm
  -> kvm_destroy_devices
   -> kvm_vfio_destroy
    -> kvm_vfio_file_set_kvm
     -> vfio_file_set_kvm
      -> group->group_lock/group_rwsem

Any suggestions for a fix?  Thanks,

Alex