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

From: David Gibson <david@gibson.dropbear.id.au>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Jason Wang <jasowang@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	Jonathan Corbet <corbet@lwn.net>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"parav@mellanox.com" <parav@mellanox.com>,
	"Alex Williamson \(alex.williamson@redhat.com\)"
	<alex.williamson@redhat.com>,
	"Enrico Weigelt, metux IT consult" <lkml@metux.net>,
	Robin Murphy <robin.murphy@arm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Shenming Lu <lushenming@huawei.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [RFC v2] /dev/iommu uAPI proposal
Date: Tue, 10 Aug 2021 16:10:12 +1000	[thread overview]
Message-ID: <YRIYRI+2qSvX/e2d@yekko> (raw)
In-Reply-To: <20210806123211.GR1721383@nvidia.com>

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On Fri, Aug 06, 2021 at 09:32:11AM -0300, Jason Gunthorpe wrote:
> On Fri, Aug 06, 2021 at 02:45:26PM +1000, David Gibson wrote:
> 
> > Well, that's kind of what I'm doing.  PCI currently has the notion of
> > "default" address space for a RID, but there's no guarantee that other
> > buses (or even future PCI extensions) will.  The idea is that
> > "endpoint" means exactly the (RID, PASID) or (SID, SSID) or whatever
> > future variations are on that.
> 
> This is already happening in this proposal, it is why I insisted that
> the driver facing API has to be very explicit. That API specifies
> exactly what the device silicon is doing.
> 
> However, that is placed at the IOASID level. There is no reason to
> create endpoint objects that are 1:1 with IOASID objects, eg for
> PASID.

They're not 1:1 though.  You can have multiple endpoints in the same
IOAS, that's the whole point.

> We need to have clear software layers and responsibilities, I think
> this is where the VFIO container design has fallen behind.
> 
> The device driver is responsible to delcare what TLPs the device it
> controls will issue

Right.. and I'm envisaging an endpoint as a abstraction to represent a
single TLP.

> The system layer is responsible to determine how those TLPs can be
> matched to IO page tables, if at all
> 
> The IO page table layer is responsible to map the TLPs to physical
> memory.
> 
> Each must stay in its box and we should not create objects that smush
> together, say, the device and system layers because it will only make
> a mess of the software design.

I agree... and endpoints are explicitly an attempt to do that.  I
don't see how you think they're smushing things together.

> Since the system layer doesn't have any concrete objects in our
> environment (which is based on devices and IO page tables) it has to
> exist as metadata attached to the other two objects.

Whereas I'm suggesting clarifying this by *creating* concrete objects
to represent the concept we need.

-- 
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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