On Tue, 18 May 2021 19:01:42 +0200
Christian Borntraeger [off-list ref] wrote:

On 18.05.21 17:33, Halil Pasic wrote:

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On Tue, 18 May 2021 15:59:36 +0200
Christian Borntraeger [off-list ref] wrote:

[..]

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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.

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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