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

Jason J. Herne
2021-05-25T13:26:10Z

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@linux.ibm.com)