From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cornelia Huck Subject: Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection Date: Wed, 23 May 2018 10:56:41 +0200 Message-ID: <20180523105641.0d89701b.cohuck@redhat.com> References: <20180518190145.3187.7620.stgit@gimli.home> <20180522123829.4e758646@w520.home> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Halil Pasic , kwankhede@nvidia.com, Dong Jia , kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Alex Williamson Return-path: In-Reply-To: <20180522123829.4e758646@w520.home> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, 22 May 2018 12:38:29 -0600 Alex Williamson wrote: > 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 No, I think your understanding is correct. We move the state from NOT_OPER to STANDBY only after we're set up completely, so our create callback will simply fail early with -ENODEV. This looks fine to me.