linux-kernel - Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach

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Date:   Mon, 14 Feb 2022 15:18:31 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...dia.com>
Cc:     Stuart Yoder <stuyoder@...il.com>, rafael@...nel.org,
        David Airlie <airlied@...ux.ie>, linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>, Ashok Raj <ashok.raj@...el.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for
 multi-device groups

On 2022-02-14 14:39, Joerg Roedel wrote:
> 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'.

Arguably, iommu_attach_device() could be renamed something like 
iommu_attach_group_for_dev(), since that's effectively the semantic that 
all the existing API users want anyway (even VFIO at the high level - 
the group is the means for the user to assign their GPU/NIC/whatever 
device to their process, not the end in itself). That's just a lot more 
churn.

It's not that callers should be blind to the entire concept of groups 
altogether - they remain a significant reason why iommu_attach_device() 
might fail, for one thing - however what callers really shouldn't need 
to be bothered with is the exact *implementation* of groups. I do 
actually quite like the idea of refining the group abstraction into 
isolation groups as a superset of alias groups, but if anything that's a 
further argument for not having the guts of the current abstraction 
exposed in places that don't need to care - otherwise that would be 
liable to be a microcosm of this series in itself: widespread churn vs. 
"same name, new meaning" compromises.

Robin.