From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cornelia Huck Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO mdev framework Date: Tue, 22 May 2018 10:19:40 +0200 Message-ID: <20180522101940.166445f8.cohuck@redhat.com> References: <1525705912-12815-1-git-send-email-akrowiak@linux.vnet.ibm.com> <1525705912-12815-6-git-send-email-akrowiak@linux.vnet.ibm.com> <5471b194-d7ca-c9c6-132e-fa9539fe44f0@linux.ibm.com> <4688078d-3e13-5201-582f-52576b25defa@linux.vnet.ibm.com> <20180517094454.6afae7ba.cohuck@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Halil Pasic , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, freude@de.ibm.com, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, borntraeger@de.ibm.com, kwankhede@nvidia.com, bjsdjshi@linux.vnet.ibm.com, pbonzini@redhat.com, alex.williamson@redhat.com, pmorel@linux.vnet.ibm.com, alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com, jjherne@linux.vnet.ibm.com, thuth@redhat.com, pasic@linux.vnet.ibm.com, berrange@redhat.com, fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com To: Tony Krowiak Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Mon, 21 May 2018 11:13:58 -0400 Tony Krowiak wrote: > On 05/17/2018 03:44 AM, Cornelia Huck wrote: > > On Mon, 14 May 2018 15:42:18 -0400 > > Tony Krowiak wrote: > > > >> On 05/11/2018 01:18 PM, Halil Pasic wrote: > >>> > >>> On 05/07/2018 05:11 PM, Tony Krowiak wrote: > >>>> Registers the matrix device created by the VFIO AP device > >>>> driver with the VFIO mediated device framework. > >>>> Registering the matrix device will create the sysfs > >>>> structures needed to create mediated matrix devices > >>>> each of which will be used to configure the AP matrix > >>>> for a guest and connect it to the VFIO AP device driver. > >>>> +static int vfio_ap_mdev_create(struct kobject *kobj, struct > >>>> mdev_device *mdev) > >>>> +{ > >>>> + struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev)); > >>>> + > >>>> + ap_matrix->available_instances--; > >>>> + > >>>> + return 0; > >>>> +} > >>>> + > >>>> +static int vfio_ap_mdev_remove(struct mdev_device *mdev) > >>>> +{ > >>>> + struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev)); > >>>> + > >>>> + ap_matrix->available_instances++; > >>>> + > >>>> + return 0; > >>>> +} > >>>> + > >>> The above functions seem to be called with the lock of this > >>> auto-generated > >>> mdev parent device held. That's why we don't have to care about > >>> synchronization > >>> ourselves, right? > >> I would assume as much. The comments for the 'struct mdev_parent_ops' in > >> include/linux/mdev.h do not mention anything about synchronization, nor > >> did I > >> see any locking or synchronization in the vfio_ccw implementation after > >> which > >> I modeled my code, so frankly it is something I did not consider. > >> > >>> > >>> A small comment in the code could be helpful for mdev non-experts. > >>> Hell, I would > >>> even consider documenting it for all mdev -- took me some time to > >>> figure out. > >> You may want to bring this up with the VFIO mdev maintainers, but I'd be > >> happy to > >> include a comment in the functions in question if you think it important. > > Important note: There's currently a patch on list that removes the mdev > > parent mutex, and it seems there was never intended to be any > > serialization in that place by the mdev core. (Look for "vfio/mdev: > > Check globally for duplicate devices".) > > The patch on the list holds the mdev_list_lock during create and remove > of an mdev device, so it looks like no synchronization is necessary on the > part of the vendor code in the create/remove callbacks; does that sound > about right? v1/v2 did that; v3/v4 hold the list lock only while the device is added to the mdev list. v4 also adds a note regarding locking to the documentation.