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

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Date:   Fri, 3 Feb 2023 08:26:44 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "jgg@...dia.com" <jgg@...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>
CC:     "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: 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?

> 
> +/**
> + * iommu_group_replace_domain - replace the domain that a group is
> attached to
> + * @new_domain: new IOMMU domain to replace with
> + * @group: IOMMU group that will be attached to the new domain
> + *
> + * This API allows the group to switch domains without being forced to go to
> + * the blocking domain in-between.
> + *
> + * If the attached domain is a core domain (e.g. a default_domain), it will act
> + * just like the iommu_attach_group().

I think you meant "the currently-attached domain", which implies a
'detached' state as you replied to Baolu.

> + */
> +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.

> +{
> +	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. Why do we need further housekeeping here?