linux-kernel - RE: [PATCH v1 2/8] iommu: Introduce a new iommu_group_replace

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Date:   Tue, 7 Feb 2023 00:32:50 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
Subject: RE: [PATCH v1 2/8] iommu: Introduce a new
 iommu_group_replace_domain() API

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Monday, February 6, 2023 9:25 PM
> 
> On Mon, Feb 06, 2023 at 06:57:35AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Friday, February 3, 2023 11:03 PM
> > >
> > > On Fri, Feb 03, 2023 at 08:26:44AM +0000, Tian, Kevin wrote:
> > > > > From: Nicolin Chen <nicolinc@...dia.com>
> > > > > Sent: Thursday, February 2, 2023 3:05 PM
> > > > >
> > > > > All drivers are already required to support changing between active
> > > > > UNMANAGED domains when using their attach_dev ops.
> > > >
> > > > All drivers which don't have *broken* UNMANAGED domain?
> > >
> > > No, all drivers.. It has always been used by VFIO.
> >
> > existing iommu_attach_group() doesn't support changing between
> > two UNMANAGED domains. only from default->unmanaged or
> > blocking->unmanaged.
> 
> Yes, but before we added the blocking domains VFIO was changing
> between unmanaged domains. Blocking domains are so new that no driver
> could have suddenly started to depend on this.

In legacy VFIO unmanaged domain was 1:1 associated with vfio
container. I didn't say how a group can switch between two
containers w/o going through transition to/from the default
domain, i.e. detach from 1st container and then attach to the 2nd.

> > btw looking at the code __iommu_group_set_domain():
> >
> > 	 * Note that this is called in error unwind paths, attaching to a
> > 	 * domain that has already been attached cannot fail.
> > 	 */
> > 	ret = __iommu_group_for_each_dev(group, new_domain,
> > 				iommu_group_do_attach_device);
> >
> > with that we don't need fall back to core domain in above error
> > unwinding per this comment.
> 
> That does make some sense.
> 
> I tried to make a patch to consolidate all this error handling once,
> that would be the better way to approach this.

that would be good.

> 
> > > In this case the API cannot retain a hidden reference to the new
> > > domain, so it must be purged, one way or another.
> >
> > Could you elaborate where the hidden reference is retained?
> 
> Inside the driver, it can keep track of the domain pointer if
> attach_dev succeeds
> 

Are you referring to no error unwinding in __iommu_group_for_each_dev()
so if it is failed some devices may have attach_dev succeeds then simply
recovering group->domain in __iommu_attach_group() is insufficient?