linux-kernel - Re: [PATCH v2 02/10] iommu: Introduce a new iommu_group_replace

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Date:   Wed, 15 Feb 2023 08:52:38 -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 v2 02/10] iommu: Introduce a new
 iommu_group_replace_domain() API

On Wed, Feb 15, 2023 at 06:10:47AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Wednesday, February 8, 2023 5:18 AM
> > 
> > +int iommu_group_replace_domain(struct iommu_group *group,
> > +			       struct iommu_domain *new_domain)
> > +{
> > +	int ret;
> > +
> > +	if (!new_domain)
> > +		return -EINVAL;
> > +
> > +	mutex_lock(&group->mutex);
> > +	ret = __iommu_group_set_domain(group, new_domain);
> > +	if (ret)
> > +		__iommu_group_set_domain(group, group->domain);
> 
> Just realize the error unwind is a nop given below:
> 
> __iommu_group_set_domain()
> {
> 	if (group->domain == new_domain)
> 		return 0;
> 
> 	...
> 
> There was an attempt [1] to fix error unwind in iommu_attach_group(), by
> temporarily set group->domain to NULL before calling set_domain().
> 
> Jason, I wonder why this recovering cannot be done in
> __iommu_group_set_domain() directly, e.g.:
> 
> 	ret = __iommu_group_for_each_dev(group, new_domain,
> 					  iommu_group_do_attach_device);
> 	if (ret) {
> 		__iommu_group_for_each_dev(group, group->domain,
> 					  iommu_group_do_attach_device);
> 		return ret;
> 	}
> 	group->domain = new_domain;

We talked about this already, some times this is not the correct
recovery case, eg if we are going to a blocking domain we need to drop
all references to the prior domain, not put them back.

Failures are WARN_ON events not error recovery.

Jason