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

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Message-ID: <Y91KAHxQIDnaGbYs@Asurada-Nvidia>
Date:   Fri, 3 Feb 2023 09:53:04 -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 Fri, Feb 03, 2023 at 11:03:20AM -0400, Jason Gunthorpe wrote:
> > > + */
> > > +int iommu_group_replace_domain(struct iommu_group *group,
> > > +			       struct iommu_domain *new_domain)
> > 
> > what actual value does 'replace' give us? It's just a wrapper of
> > __iommu_group_set_domain() then calling it set_domain is
> > probably clearer. You can clarify the 'replace' behavior in the
> > comment.
> 
> As the APIs are setup:
> 
> attach demands that no domain is currently set (eg the domain must be blocking)
> 
> replace permits the domain to be currently set
> 
> 'set' vs 'attach' is really unclear what the intended difference is.
> 
> We could also address this by simply removing the protection from
> attach, but it is not so clear if that is safe for the few users.

I can add a couple of lines to the commit message to make things
clear.

> > > +{
> > > +	int ret;
> > > +
> > > +	if (!new_domain)
> > > +		return -EINVAL;
> > > +
> > > +	mutex_lock(&group->mutex);
> > > +	ret = __iommu_group_set_domain(group, new_domain);
> > > +	if (ret) {
> > > +		if (__iommu_group_set_domain(group, group->domain))
> > > +			__iommu_group_set_core_domain(group);
> > > +	}
> > 
> > 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.
> 
> In this case the API cannot retain a hidden reference to the new
> domain, so it must be purged, one way or another.

Considering it is a bit different than the existing APIs like
iommu_attach_group(), perhaps I should add a line of comments
in the fallback routine.

Thanks
Nic