On Tue, 18 May 2021 19:01:42 +0200 Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: > On 18.05.21 17:33, Halil Pasic wrote: > > On Tue, 18 May 2021 15:59:36 +0200 > > Christian Borntraeger <borntraeger@xxxxxxxxxx> 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
