Re: [RFC v2] /dev/iommu uAPI proposal

From: David Gibson <david@gibson.dropbear.id.au>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"Alex Williamson (alex.williamson@redhat.com)" 
	<alex.williamson@redhat.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Jason Wang <jasowang@redhat.com>,
	"parav@mellanox.com" <parav@mellanox.com>,
	"Enrico Weigelt, metux IT consult" <lkml@metux.net>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Shenming Lu <lushenming@huawei.com>,
	Joerg Roedel <joro@8bytes.org>,
	Eric Auger <eric.auger@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	"Liu, Yi L" <yi.l.liu@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	Robin Murphy <robin.murphy@arm.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Lu Baolu <baolu.lu@linux.intel.com>
Subject: Re: [RFC v2] /dev/iommu uAPI proposal
Date: Fri, 6 Aug 2021 14:24:28 +1000	[thread overview]
Message-ID: <YQy5fBJkKHhuNUuP@yekko> (raw)
In-Reply-To: <20210804140742.GI1721383@nvidia.com>

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On Wed, Aug 04, 2021 at 11:07:42AM -0300, Jason Gunthorpe wrote:
> On Tue, Aug 03, 2021 at 11:58:54AM +1000, David Gibson wrote:
> > > I'd rather deduce the endpoint from a collection of devices than the
> > > other way around...
> > 
> > Which I think is confusing, and in any case doesn't cover the case of
> > one "device" with multiple endpoints.
> 
> Well they are both confusing, and I'd prefer to focus on the common
> case without extra mandatory steps. Exposing optional endpoint sharing
> information seems more in line with where everything is going than
> making endpoint sharing a first class object.
> 
> AFAIK a device with multiple endpoints where those endpoints are
> shared with other devices doesn't really exist/or is useful? Eg PASID
> has multiple RIDs by they are not shared.

No, I can't think of a (non-contrived) example where a device would
have *both* multiple endpoints and those endpoints are shared amongst
multiple devices.  I can easily think of examples where a device has
multiple (non shared) endpoints and where multiple devices share a
single endpoint.

The point is that making endpoints explicit separates the various
options here from the logic of the IOMMU layer itself.  New device
types with new possibilities here means new interfaces *on those
devices*, but not new interfaces on /dev/iommu.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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