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

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Date:   Thu, 3 Jun 2021 08:52:24 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Gibson <david@...son.dropbear.id.au>
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 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"

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