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

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Date:   Tue, 25 May 2021 09:24:59 -0400
From:   "Jason J. Herne" <jjherne@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com
Subject: Re: [PATCH v4 2/2] s390/vfio-ap: control access to PQAP(AQIC)
 interception handler

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@...ux.ibm.com)