Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for multi-device groups

From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, Jason Gunthorpe <jgg@nvidia.com>
Cc: Stuart Yoder <stuyoder@gmail.com>,
	rafael@kernel.org, David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Ashok Raj <ashok.raj@intel.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for multi-device groups
Date: Mon, 14 Feb 2022 15:18:31 +0000	[thread overview]
Message-ID: <08e90a61-8491-acf1-ab0f-f93f97366d24@arm.com> (raw)
In-Reply-To: <Ygppub+Wjq6mQEAX@8bytes.org>

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.