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

David Gibson
2021-08-10T05:50:03Z

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On Mon, Aug 09, 2021 at 08:34:06AM +0000, Tian, Kevin wrote:
> > From: David Gibson <david@gibson.dropbear.id.au>
> > Sent: Friday, August 6, 2021 12:45 PM
> > > > > In concept I feel the purpose of DMA endpoint is equivalent to the
> > routing
> > > > > info in this proposal.
> > > >
> > > > Maybe? I'm afraid I never quite managed to understand the role of the
> > > > routing info in your proposal.
> > > >
> > >
> > > the IOMMU routes incoming DMA packets to a specific I/O page table,
> > > according to RID or RID+PASID carried in the packet. RID or RID+PASID
> > > is the routing information (represented by device cookie +PASID in
> > > proposed uAPI) and what the iommu driver really cares when activating
> > > the I/O page table in the iommu.
> >
> > Ok, so yes, endpoint is roughly equivalent to that. But my point is
> > that the IOMMU layer really only cares about that (device+routing)
> > combination, not other aspects of what the device is. So that's the
> > concept we should give a name and put front and center in the API.
> >
>
> This is how this proposal works, centered around the routing info. the
> uAPI doesn't care what the device is. It just requires the user to specify
> the user view of routing info (device fd + optional pasid) to tag an IOAS.

Which works as long as (just device fd) and (device fd + PASID) covers
all the options. If we have new possibilities we need new interfaces.
And, that can't even handle the case of one endpoint for multiple
devices (e.g. ACS-incapable bridge).

> the user view is then converted to the kernel view of routing (rid or
> rid+pasid) by vfio driver and then passed to iommu fd in the attaching
> operation. and GET_INFO interface is provided for the user to check
> whether a device supports multiple IOASes and whether pasid space is
> delegated to the user. We just need a better name if pasid is considered
> too pci specific...
>
> But creating an endpoint per ioasid and making it centered in uAPI is not
> what the IOMMU layer cares about.

It's not an endpoint per ioasid. You can have multiple endpoints per
ioasid, just not the other way around. As it is multiple IOASes per
device means *some* sort of disambiguation (generally by PASID) which
is hard to describe generall. Having endpoints as a first-class
concept makes that simpler.

--
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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