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 09:16:30 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     jjherne@...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.

That would be great, however; when I implemented something similar, it 
resulted in a
lockdep splat between the lock used to protect the hook and the 
matrix_dev->lock used to
protect updates to matrix_mdev (including the freeing thereof). After 
pulling what little hair
I have left out, this seemed like a reasonable solution, over-engineered 
though it may be.
If somebody has a simpler solution, I'm all ears.

>
> 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.
>