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

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Date:   Tue, 8 Jun 2021 10:53:02 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Jason Wang <jasowang@...hat.com>,
        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>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Thu, Jun 03, 2021 at 08:52:24AM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 03, 2021 at 03:13:44PM +1000, David Gibson wrote:
> 
> > > We can still consider it a single "address space" from the IOMMU
> > > perspective. What has happened is that the address table is not just a
> > > 64 bit IOVA, but an extended ~80 bit IOVA formed by "PASID, IOVA".
> > 
> > True.  This does complexify how we represent what IOVA ranges are
> > valid, though.  I'll bet you most implementations don't actually
> > implement a full 64-bit IOVA, which means we effectively have a large
> > number of windows from (0..max IOVA) for each valid pasid.  This adds
> > another reason I don't think my concept of IOVA windows is just a
> > power specific thing.
> 
> Yes
> 
> Things rapidly get into weird hardware specific stuff though, the
> request will be for things like:
>   "ARM PASID&IO page table format from SMMU IP block vXX"

So, I'm happy enough for picking a user-managed pagetable format to
imply the set of valid IOVA ranges (though a query might be nice).

I'm mostly thinking of representing (and/or choosing) valid IOVA
ranges as something for the kernel-managed pagetable style
(MAP/UNMAP).

> Which may have a bunch of (possibly very weird!) format specific data
> to describe and/or configure it.
> 
> The uAPI needs to be suitably general here. :(
> 
> > > If we are already going in the direction of having the IOASID specify
> > > the page table format and other details, specifying that the page
> > > tabnle format is the 80 bit "PASID, IOVA" format is a fairly small
> > > step.
> > 
> > Well, rather I think userspace needs to request what page table format
> > it wants and the kernel tells it whether it can oblige or not.
> 
> Yes, this is what I ment.
> 
> Jason
> 

-- 
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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