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

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Message-ID: <Y+KjgdNd4gi+6R05@Asurada-Nvidia>
Date:   Tue, 7 Feb 2023 11:16:17 -0800
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Tian, Kevin" <kevin.tian@...el.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 09:25:17AM -0400, Jason Gunthorpe wrote:

> > > > 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..

I'd expect the doc to come with some other patch/series than this
replace series, so I think we should be fine without adding a line
of comments in this patch?

> > 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.

Then, I'll drop the core-domain line. Combining my reply above:

+	mutex_lock(&group->mutex);
+	ret = __iommu_group_set_domain(group, new_domain);
+	if (ret)
+		__iommu_group_set_domain(group, group->domain);
+	mutex_unlock(&group->mutex);

Will wrap things up and send v2 today.

Thanks
Nic