linux-kernel - RE: [PATCH v3 03/15] iommu: Always register bus notifiers

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Date:   Fri, 8 Jul 2022 05:50:12 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        "joro@...tes.org" <joro@...tes.org>
CC:     "will@...nel.org" <will@...nel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "vasant.hegde@....com" <vasant.hegde@....com>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "gerald.schaefer@...ux.ibm.com" <gerald.schaefer@...ux.ibm.com>,
        "schnelle@...ux.ibm.com" <schnelle@...ux.ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 03/15] iommu: Always register bus notifiers

> From: Robin Murphy <robin.murphy@....com>
> Sent: Thursday, July 7, 2022 5:59 PM
> 
> On 2022-07-07 07:31, Tian, Kevin wrote:
> >> From: Robin Murphy <robin.murphy@....com>
> >> Sent: Wednesday, July 6, 2022 1:08 AM
> >>
> >> The number of bus types that the IOMMU subsystem deals with is small
> and
> >> manageable, so pull that list into core code as a first step towards
> >> cleaning up all the boilerplate bus-awareness from drivers. Calling
> >> iommu_probe_device() before bus->iommu_ops is set will simply return
> >> -ENODEV and not break the notifier call chain, so there should be no
> >> harm in proactively registering all our bus notifiers at init time.
> >>
> >
> > Suppose we miss a check on iommu ops in iommu_release_device():
> >
> > 	if (!dev->iommu) <<<<<<<
> > 		return;
> >
> > 	iommu_device_unlink(dev->iommu->iommu_dev, dev);
> >
> > 	ops = dev_iommu_ops(dev);
> > 	ops->release_device(dev);
> >
> > following the rationale in patch01 a device could be removed when
> > it's associated with a known but not registered instance.
> 
> No, because at that point the instance is only known internally to the
> driver. As long as it isn't erroneously returned from
> ->probe_device(dev), dev->iommu will remain NULL and the rest of the
> core code works as expected.
> 

You are correct. I overlooked dev->iommu as device_to_iommu() in
patch01. As long as the device hasn't been probed or ->probe_device
doesn't do bad thing then dev->iommu should be NULL in this case.