From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Williamson Subject: Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection Date: Tue, 22 May 2018 12:38:29 -0600 Message-ID: <20180522123829.4e758646@w520.home> References: <20180518190145.3187.7620.stgit@gimli.home> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: kwankhede@nvidia.com, Dong Jia , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cohuck@redhat.com To: Halil Pasic Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, 22 May 2018 19:17:07 +0200 Halil Pasic wrote: > From vfio-ccw perspective I join Connie's assessment: vfio-ccw should > be fine with these changes. I'm however not too deeply involved with > the mdev framework, thus I don't feel comfortable r-b-ing. That results > in > Acked-by: Halil Pasic > for both patches. > > While at it I have would like to ask about the semantics and intended > use of the mdev interfaces. > > static int vfio_ccw_sch_probe(struct subchannel *sch) > { > > /* HALIL: 8< Not so interesting stuff happens here. >8 */ This was interesting: private->state = VFIO_CCW_STATE_NOT_OPER; > ret = vfio_ccw_mdev_reg(sch); > if (ret) > goto out_disable; > /* > * HALIL: > * This might be racy. Somewhere in vfio_ccw_mdev_reg() the create attribute > * is made available (it calls mdev_register_device()). For instance create will > * attempt to decrement private->avail which is initialized below. I fail to > * understand how is this well synchronized. > */ > INIT_WORK(&private->io_work, vfio_ccw_sch_io_todo); > atomic_set(&private->avail, 1); > private->state = VFIO_CCW_STATE_STANDBY; > > return 0; > > out_disable: > cio_disable_subchannel(sch); > out_free: > dev_set_drvdata(&sch->dev, NULL); > kfree(private); > return ret; > } > > Should not initialization of go before mdev_register_device(), and then rolled > back if necessary if mdev_register_device() fails? > > In practice it does not seem very likely that userspace can trigger > mdev_device_create() before vfio_ccw_sch_probe() finishes so it should > not be a practical problem. But I would like to understand how synchronization > is supposed to work. > > [Added Dong Jia, maybe he is also able to answer my question.] vfio_ccw_mdev_create() requires that private->state is not VFIO_CCW_STATE_NOT_OPER but vfio_ccw_sch_probe() explicitly sets state to this value before calling vfio_ccw_mdev_reg(), so a create should return -ENODEV if racing with parent registration. Is there something else that I'm missing? Thanks, Alex