On Tue, 18 May 2021 19:01:42 +0200 Christian Borntraeger borntraeger@de.ibm.com wrote:
On 18.05.21 17:33, Halil Pasic wrote:
On Tue, 18 May 2021 15:59:36 +0200 Christian Borntraeger borntraeger@de.ibm.com wrote:
[..]
Would it help, if the code in priv.c would read the hook once and then only work on the copy? We could protect that with rcu and do a synchronize rcu in vfio_ap_mdev_unset_kvm after unsetting the pointer?
Unfortunately just "the hook" is ambiguous in this context. We have kvm->arch.crypto.pqap_hook that is supposed to point to a struct kvm_s390_module_hook member of struct ap_matrix_mdev which is also called pqap_hook. And struct kvm_s390_module_hook has function pointer member named "hook".
I was referring to the full struct.
I'll look into this.
I think it could work. in priv.c use rcu_readlock, save the pointer, do the check and call, call rcu_read_unlock. In vfio_ap use rcu_assign_pointer to set the pointer and after setting it to zero call sychronize_rcu.
In my opinion, we should make the accesses to the kvm->arch.crypto.pqap_hook pointer properly synchronized. I'm not sure if that is what you are proposing. How do we usually do synchronisation on the stuff that lives in kvm->arch?
RCU is a method of synchronization. We make sure that structure pqap_hook is still valid as long as we are inside the rcu read lock. So the idea is: clear pointer, wait until all old readers have finished and the proceed with getting rid of the structure.
Yes I know that RCU is a method of synchronization, but I'm not very familiar with it. I'm a little confused by "read the hook once and then work on a copy". I guess, I would have to read up on the RCU again to get clarity. I intend to brush up my RCU knowledge once the patch comes along. I would be glad to have your help when reviewing an RCU based solution for this.
Regards, Halil