Re: [PATCH v4 2/2] s390/vfio-ap: control access to PQAP(AQIC) interception handler

[Jason J. Herne]

On 5/24/21 10:37 AM, Jason J. Herne wrote:
> On 5/21/21 3:36 PM, Tony Krowiak wrote:
> > The function pointer to the handler that processes interception of the
> > PQAP instruction is contained in the mdev. If the mdev is removed and
> > its storage de-allocated during the processing of the PQAP instruction,
> > the function pointer could get wiped out before the function is called
> > because there is currently nothing that controls access to it.
> >
> > This patch introduces two new functions:
> > * The kvm_arch_crypto_register_hook() function registers a function pointer
> >    for processing intercepted crypto instructions.
> > * The kvm_arch_crypto_register_hook() function un-registers a function
> >    pointer that was previously registered.
>
> Typo: You meant kvm_arch_crypto_UNregister_hook() in the second bullet.
>
>
> Just one overall observation on this one. The whole hook system seems kind of 
> over-engineered if this is our only use for it. It looks like a kvm_s390_crypto_hook is 
> meant to link a specific module with a function pointer. Do we really need this concept?
>
> I think a simpler design could be to just place a mutex and a function pointer in the 
> kvm_s390_crypto struct. Then you can grab the mutex in vfio_ap_ops.c when 
> registering/unregistering. You would also grab the mutex in priv.c when calling the 
> function pointer. What I am suggesting is essentially the exact same scheme you have 
> implemented here, but simpler and with less infrastructure.
>
> With that said, I'll point out that I am relative new to this code (and this patch series) 
> so maybe I've missed something and the extra complexity is needed for some reason. But if 
> it is not, I'm all in favor of keeping things simple.

After thinking about this problem a bit more, I'm wondering if we can remove the lock 
entirely. How about we store a function pointer in kvm_s390_crypto? Initially that 
function pointer will point to a stub function that handles the error case, exactly like 
it is done in priv.c:handle_pqap() today when the function pointer would be NULL. When the 
ap module loads, we can simply change the function pointer to point to 
vfio_ap_ops:handle_pqap(). When we unload the module we change the function pointer back 
to the stub.  The updates should be atomic operations so no lock needed, right?

--
-- Jason J. Herne (jjherne@xxxxxxxxxxxxx)