On 19.05.21 01:27, Halil Pasic wrote:
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.
Just had a quick look. Its not trivial, as the hook function itself
takes a mutex and an rcu section must not sleep. Will have a deeper
look.
