Re: [RFC] /dev/ioasid uAPI proposal

David Gibson
2021-06-08T02:26:04Z

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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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