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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     David Gibson <david@gibson.dropbear.id.au>
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Subject: RE: [RFC v2] /dev/iommu uAPI proposal
Thread-Topic: [RFC v2] /dev/iommu uAPI proposal
Thread-Index: Add0lrMH87IsTsl5Rp6WN1oQU6kGMQNQxoYAAGGEwqABKoT0AAAAch4QAJyI0wAAnMDPwAAsfuqAAAI4CRA=
Date:   Tue, 10 Aug 2021 06:04:21 +0000
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> From: David Gibson <david@gibson.dropbear.id.au>
> Sent: Tuesday, August 10, 2021 12:48 PM
>=20
> 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 o=
f
> 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+PAS=
ID
> > > > is the routing information (represented by device cookie +PASID in
> > > > proposed uAPI) and what the iommu driver really cares when activati=
ng
> > > > 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 spec=
ify
> > the user view of routing info (device fd + optional pasid) to tag an IO=
AS.
>=20
> 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).

why not? We have went through a long debate in v1 to reach conclusion
that a device-centric uAPI can cover above group scenario (see section 1.3)

>=20
> > 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 considere=
d
> > too pci specific...
> >
> > But creating an endpoint per ioasid and making it centered in uAPI is n=
ot
> > what the IOMMU layer cares about.
>=20
> 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

you need create one endpoint per device fd to attach to gpa_ioasid.

then one endpoint per device fd to attach to pasidtbl_ioasid on arm/amd.

then one endpoint per pasid to attach to gva_ioasid on intel.

In the end you just create one endpoint per each attached ioasid given=20
a device.

> 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.
>=20

I don't think pasid causes any disambiguation (except the name itself
being pci specific). with multiple IOASes you always need an id to tag it.=
=20
This id is what iommu layer cares about. which endpoint on the device=20
uses the id is not a business to iommu.

Thanks
Kevin