On Wed, May 19, 2021 at 01:22:56PM +0200, Christian Borntraeger wrote:
On 19.05.21 10:17, Christian Borntraeger wrote:
On 19.05.21 01:27, Halil Pasic wrote:
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.
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.
As a quick hack something like this could work. The whole locking is pretty complicated and this makes it even more complex so we might want to do a cleanup/locking rework later on.
index 9928f785c677..fde6e02aab54 100644 +++ b/arch/s390/kvm/priv.c @@ -609,6 +609,7 @@ static int handle_io_inst(struct kvm_vcpu *vcpu) */ static int handle_pqap(struct kvm_vcpu *vcpu) {
struct kvm_s390_module_hook *pqap_hook; struct ap_queue_status status = {}; unsigned long reg0; int ret;@@ -657,14 +658,21 @@ static int handle_pqap(struct kvm_vcpu *vcpu) * Verify that the hook callback is registered, lock the owner * and call the hook. */
if (vcpu->kvm->arch.crypto.pqap_hook) {if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
rcu_read_lock();pqap_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);if (pqap_hook) {if (!try_module_get(pqap_hook->owner)) {
module locking doesn't prevent driver unbinding
rcu_read_unlock(); return -EOPNOTSUPP;
ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);
}rcu_read_unlock();ret = pqap_hook->hook(vcpu);
So taking the pointer out of the rcu still isn't protected.
Unless this is super performance critical just use a rw sem
Jason