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Thu, 01 Sep 2022 20:29:10 +0000 Received: from b03ledav001.gho.boulder.ibm.com (b03ledav001.gho.boulder.ibm.com [9.17.130.232]) by b03cxnp08026.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 281KT9DQ9437912 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 1 Sep 2022 20:29:09 GMT Received: from b03ledav001.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 190356E054; Thu, 1 Sep 2022 20:29:09 +0000 (GMT) Received: from b03ledav001.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BBFBC6E070; Thu, 1 Sep 2022 20:29:07 +0000 (GMT) Received: from [9.160.86.252] (unknown [9.160.86.252]) by b03ledav001.gho.boulder.ibm.com (Postfix) with ESMTP; Thu, 1 Sep 2022 20:29:07 +0000 (GMT) Message-ID: <6be7b0ff-63d4-0352-a7de-e66a93411c2b@linux.ibm.com> Date: Thu, 1 Sep 2022 16:28:35 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.0 Subject: Re: [PATCH v4 1/2] iommu/s390: Fix race with release_device ops Content-Language: en-US To: Niklas Schnelle , Pierre Morel , iommu@lists.linux.dev Cc: linux-s390@vger.kernel.org, borntraeger@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com, gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com, svens@linux.ibm.com, joro@8bytes.org, will@kernel.org, robin.murphy@arm.com, jgg@nvidia.com, linux-kernel@vger.kernel.org References: <20220831201236.77595-1-mjrosato@linux.ibm.com> <20220831201236.77595-2-mjrosato@linux.ibm.com> <9887e2f4-3f3d-137d-dad7-59dab5f98aab@linux.ibm.com> <52d3fe0b86bdc04fdbf3aae095b2f71f4ea12d44.camel@linux.ibm.com> From: Matthew Rosato In-Reply-To: <52d3fe0b86bdc04fdbf3aae095b2f71f4ea12d44.camel@linux.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 3G1ovyeNmRMDx_iHQNld8e02hC9irtkm X-Proofpoint-GUID: 3G1ovyeNmRMDx_iHQNld8e02hC9irtkm X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.517,FMLib:17.11.122.1 definitions=2022-09-01_12,2022-08-31_03,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 adultscore=0 phishscore=0 clxscore=1015 mlxscore=0 spamscore=0 suspectscore=0 lowpriorityscore=0 malwarescore=0 mlxlogscore=999 bulkscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2207270000 definitions=main-2209010087 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On 9/1/22 5:37 AM, Niklas Schnelle wrote: > On Thu, 2022-09-01 at 09:56 +0200, Pierre Morel wrote: >> >> On 8/31/22 22:12, Matthew Rosato wrote: >>> With commit fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev >>> calls") s390-iommu is supposed to handle dynamic switching between IOMMU >>> domains and the DMA API handling. However, this commit does not >>> sufficiently handle the case where the device is released via a call >>> to the release_device op as it may occur at the same time as an opposing >>> attach_dev or detach_dev since the group mutex is not held over >>> release_device. This was observed if the device is deconfigured during a >>> small window during vfio-pci initialization and can result in WARNs and >>> potential kernel panics. >>> >>> Handle this by tracking when the device is probed/released via >>> dev_iommu_priv_set/get(). Ensure that once the device is released only >>> release_device handles the re-init of the device DMA. >>> >>> Fixes: fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev calls") >>> Signed-off-by: Matthew Rosato >>> --- >>> arch/s390/include/asm/pci.h | 1 + >>> arch/s390/pci/pci.c | 1 + >>> drivers/iommu/s390-iommu.c | 39 ++++++++++++++++++++++++++++++++++--- >>> 3 files changed, 38 insertions(+), 3 deletions(-) >>> >>> > ---8<--- >>> >>> @@ -206,10 +221,28 @@ static void s390_iommu_release_device(struct device *dev) >>> >> > ---8<--- >>> + /* Make sure this device is removed from the domain list */ >>> domain = iommu_get_domain_for_dev(dev); >>> if (domain) >>> s390_iommu_detach_device(domain, dev); >>> + /* Now ensure DMA is initialized from here */ >>> + mutex_lock(&zdev->dma_domain_lock); >>> + if (zdev->s390_domain) { >>> + zdev->s390_domain = NULL; >>> + zpci_unregister_ioat(zdev, 0); >>> + zpci_dma_init_device(zdev); >> >> Sorry if it is a stupid question, but two things looks strange to me: >> >> - having DMA initialized just after having unregistered the IOAT >> Is that really all we need to unregister before calling dma_init_device? This is also how s390-iommu has been handling detach_dev (and still does) >> >> - having DMA initialized inside the release_device callback: >> Why isn't it done in the device_probe ? > > As I understand it iommu_release_device() which calls this code is only > used when a device goes away. So, I think you're right in that it makes > little sense to re-initialize DMA at this point, it's going to be torn > down immediately after anyway. I do wonder if it would be an acceptably > small change to just set zdev->s390_domain = NULL here and leave DMA > uninitialized while making zpci_dma_exit_device() deal with that e.g. > by doing nothing if zdev->dma_table is NULL but I'm not sure. Right -- since it's a fix, I was trying to keep the changes minimal and this behavior (re-init DMA even on release_device) was existing, it was just always done within s390_iommu_detach_device before. If you want, I could experiment with setting zdev->dma_table = NULL on the release path only (and checking it in zpci_dma_exit_device()) > > Either way I fear this mess really is just a symptom of our current > design oddity of driving the same IOMMU hardware through both our DMA > API implementation (arch/s390/pci_dma.c) and the IOMMU driver > (driver/iommu/s390-iommu.c) and trying to hand off between them > smoothly where common code instead just layers one atop the other when > using an IOMMU at all. > > I think the correct medium term solution is to use the common DMA API > implementation (drivers/iommu/dma-iommu.c) like everyone else. But that > isn't the minimal fix we need now. Agree > > I do have a working prototype of using the common implementation but > the big problem that I'm still searching a solution for is its > performance with a virtualized IOMMU where IOTLB flushes (RPCIT on > s390) are used for shadowing and are expensive and serialized. The > optimization we used so far for unmap, only doing one global IOTLB > flush once we run out of IOVA space, is just too much better in that > scenario to just ignore. As one data point, on an NVMe I get about > _twice_ the IOPS when using our existing scheme compared to strict > mode. Which makes sense as IOTLB flushes are known as the bottleneck > and optimizing unmap like that reduces them by almost half. Queued > flushing is still much worse likely due to serialization of the > shadowing, though again it works great on LPAR. To make sure it's not > due to some bug in the IOMMU driver I even tried converting our > existing DMA driver to layer on top of the IOMMU driver with the same > result. > >