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

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jul 30, 2021 at 11:51:23AM -0300, Jason Gunthorpe wrote:
> On Mon, Jul 26, 2021 at 02:50:48PM +1000, David Gibson wrote:
> 
> > That said, I'm still finding the various ways a device can attach to
> > an ioasid pretty confusing.  Here are some thoughts on some extra
> > concepts that might make it easier to handle [note, I haven't thought
> > this all the way through so far, so there might be fatal problems with
> > this approach].
> 
> I think you've summarized how I've been viewing this problem. All the
> concepts you pointed to should show through in the various APIs at the
> end, one way or another.
> 
> How much we need to expose to userspace, I don't know.
> 
> Does userspace need to care how the system labels traffic between DMA
> endpoint and the IOASID? At some point maybe yes since stuff like
> PASID does leak out in various spots

Yeah, I'm not sure.  I think it probably doesn't for the "main path"
of the API, though we might want to expose that for debugging and some
edge cases.

We *should* however be exposing the address type for each IOAS, since
that affects how your MAP operations will work, as well as what
endpoints are compatible with the IOAS.

> > /dev/iommu would work entirely (or nearly so) in terms of endpoint
> > handles, not device handles.  Endpoints are what get bound to an IOAS,
> > and endpoints are what get the user chosen endpoint cookie.
> 
> While an accurate modeling of groups, it feels like an
> overcomplication at this point in history where new HW largely doesn't
> need it.

So.. first, is that really true across the board?  I expect it's true
of high end server hardware, but for consumer level and embedded
hardware as well?  Then there's virtual hardware - I could point to
several things still routinely using emulated PCIe to PCI bridges in
qemu.

Second, we can't just ignore older hardware.

> The user interface VFIO and others presents is device
> centric, inserting a new endpoint object is going going back to some
> kind of group centric view of the world.

Well, kind of, yeah, because I still think the concept has value.
Part of the trouble is that "device" is pretty ambiguous.  "Device" in
the sense of PCI address for register interface may not be the same as
"device" in terms of DMA RID may not be the same as as "device" in
terms of Linux struct device 


terms of PCI register interface is not the same as "device"
in terms of RID / DMA identifiability is not the same "device" in
terms of what.

> I'd rather deduce the endpoint from a collection of devices than the
> other way around...

Which I think is confusing, and in any case doesn't cover the case of
one "device" with multiple endpoints.

-- 
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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