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

Alex Williamson
2023-01-05T22:10:46Z

On Thu, 19 May 2022 14:33:11 -0400
Matthew Rosato <mjrosato@linux.ibm.com> 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