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Date:   Mon, 14 Feb 2022 15:39:53 +0100
From:   Joerg Roedel <joro@8bytes.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Lu Baolu <baolu.lu@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
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Subject: Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for
 multi-device groups
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References: <20220106022053.2406748-1-baolu.lu@linux.intel.com>
 <20220106022053.2406748-4-baolu.lu@linux.intel.com>
 <Ygo/eCRFnraY01WA@8bytes.org>
 <20220214130313.GV4160@nvidia.com>
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On Mon, Feb 14, 2022 at 09:03:13AM -0400, Jason Gunthorpe wrote:
> Groups should disappear into an internal implementation detail, not be
> so prominent in the API.

Not going to happen, IOMMU groups are ABI and todays device assignment
code, including user-space, relies on them.

Groups implement and important aspect of hardware IOMMUs that the API
can not abstract away: That there are devices which share the same
request-id. 

This is not an issue for devices concerned by iommufd, but for legacy
device assignment it is. The IOMMU-API needs to handle both in a clean
API, even if it means that drivers need to lookup the sub-group of a
device first.

And I don't see how a per-device API can handle both in a device-centric
way. For sure it is not making it 'device centric but operate on groups
under the hood'.

Regards,

	Joerg