linux-kernel - Re: [RFC v2] /dev/iommu uAPI proposal

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Date:   Tue, 10 Aug 2021 16:10:12 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shenming Lu <lushenming@...wei.com>,
        Joerg Roedel <joro@...tes.org>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: [RFC v2] /dev/iommu uAPI proposal

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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