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

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Date:   Mon, 6 Feb 2023 09:25:17 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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

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.

> > > Can you elaborate the error handling here? Ideally if
> > > __iommu_group_set_domain() fails then group->domain shouldn't
> > > be changed.
> > 
> > That isn't what it implements though. The internal helper leaves
> > things in a mess, it is for the caller to fix it, and it depends on
> > the caller what that means.
> 
> I didn't see any warning of the mess and the caller's responsibility
> in __iommu_group_set_domain(). Can it be documented clearly
> so if someone wants to add a new caller on it he can clearly know
> what to do?

That would be nice..
 
> and why doesn't iommu_attach_group() need to do anything
> when an attach attempt fails? In the end it calls the same
> iommu_group_do_attach_device() as __iommu_group_set_domain()
> does.

That's a bug for sure.

 
> 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.

> > 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

Jason