Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, borntraeger@de.ibm.com,
        cohuck@redhat.com, pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com, david@redhat.com,
        pbonzini@redhat.com, frankja@linux.ibm.com, imbrenda@linux.ibm.com
References: <20210719193503.793910-1-akrowiak@linux.ibm.com>
 <20210719193503.793910-3-akrowiak@linux.ibm.com>
 <20210721164550.5402fe1c.pasic@linux.ibm.com>
 <c3b80f79-6795-61ce-2dd1-f4cc7110e417@linux.ibm.com>
 <20210723162625.59cead27.pasic@linux.ibm.com>
 <5380652f-e68f-bbd0-10c0-c7d541065843@linux.ibm.com>
 <20210726223628.4d7759bf.pasic@linux.ibm.com>
 <20210726220317.GA1721383@nvidia.com>
 <20210727004329.3bcc7d4f.pasic@linux.ibm.com>
 <a5eeac87-069d-171b-5558-3e99e7bda539@linux.ibm.com>
 <20210728214257.5e5c28c4.pasic@linux.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <8e0b4b16-ce45-449e-4849-29919da3a70f@linux.ibm.com>
Date:   Fri, 30 Jul 2021 09:33:09 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.11.0
MIME-Version: 1.0
In-Reply-To: <20210728214257.5e5c28c4.pasic@linux.ibm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk


On 7/28/21 3:42 PM, Halil Pasic wrote:
> On Wed, 28 Jul 2021 09:43:03 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> This solution was suggested by Jason G and it does in fact resolve
>> the lockdep splat encountered when starting an SE guest with
>> access to crypto resources. There is a chance that the KVM lock
>> can get held while waiting for the lock on the matrix_dev->mutex,
>> but this does not seem like a grave concern to me.
> Yes I agree. I was thinking along the lines: matrix modifications
> via the sysfs take the matrix_dev->lock so the level of contention
> may depend on what userspace is doing...

The probe/remove functions also take the matrix_dev->lock
as does the handle_pqap() function. In any case, while all of
those are possible, in our implementation of AP queue
pass-through, the two functions that take the KVM lock
are invoked when the guest is starting or shutting down,
or when the mdev is hot plugged/unplugged. For the cases of
guest startup/shutdown, it would seem that holding the
kvm->lock while waiting for the matrix_dev->lock shouldn't
be a big problem since the guest will either not be fully up
yet or on its way down. I suppose the hot plug/unplug case
could potentially cause the guest vcpus to pause while processing,
but how often do you anticipate a hot plug to take place?

>
> Regards,
> Halil