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Date:   Tue, 25 May 2021 10:19:12 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Tony Krowiak <akrowiak@linux.ibm.com>
Cc:     jjherne@linux.ibm.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, borntraeger@de.ibm.com,
        cohuck@redhat.com, pasic@linux.vnet.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
        hca@linux.ibm.com
Subject: Re: [PATCH v4 2/2] s390/vfio-ap: control access to PQAP(AQIC)
 interception handler
Message-ID: <20210525131912.GW1002214@nvidia.com>
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On Tue, May 25, 2021 at 09:16:30AM -0400, Tony Krowiak wrote:
> 
> 
> 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.

Why can't you put the locks in the right order? It looked trivial, I'm confused.

Jason