Re: Plan for /dev/ioasid RFC v2

David Gibson
2021-06-17T07:22:35Z

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On Wed, Jun 09, 2021 at 09:39:19AM -0300, Jason Gunthorpe wrote:
> On Wed, Jun 09, 2021 at 02:24:03PM +0200, Joerg Roedel wrote:
> > On Mon, Jun 07, 2021 at 02:58:18AM +0000, Tian, Kevin wrote:
> > > - Device-centric (Jason) vs. group-centric (David) uAPI. David is not fully
> > > convinced yet. Based on discussion v2 will continue to have ioasid uAPI
> > > being device-centric (but it's fine for vfio to be group-centric). A new
> > > section will be added to elaborate this part;
> >
> > I would vote for group-centric here. Or do the reasons for which VFIO is
> > group-centric not apply to IOASID? If so, why?
>
> VFIO being group centric has made it very ugly/difficult to inject
> device driver specific knowledge into the scheme.
>
> The device driver is the only thing that knows to ask:
> - I need a SW table for this ioasid because I am like a mdev
> - I will issue TLPs with PASID
> - I need a IOASID linked to a PASID
> - I am a devices that uses ENQCMD and vPASID
> - etc in future

mdev drivers might know these, but shim drivers, like basic vfio-pci
often won't. In that case only the userspace driver will know that
for certain. The shim driver at best has a fairly loose bound on what
the userspace driver *could* do.

I still think you're having a tendency to partially conflate several
meanings of "group":
1. the unavoidable hardware unit of non-isolation
2. the kernel internal concept and interface to it
3. the user visible fd and interface

We can't avoid having (1) somewhere, (3) and to a lesser extent (2)
are what you object to.

> The current approach has the group try to guess the device driver
> intention in the vfio type 1 code.

I agree this has gotten ugly. What I'm not yet convinced of is that
reworking groups to make this not-ugly necessarily requires totally
minimizing the importance of groups.

> I want to see this be clean and have the device driver directly tell
> the iommu layer what kind of DMA it plans to do, and thus how it needs
> the IOMMU and IOASID configured.

>
> This is the source of the ugly symbol_get and the very, very hacky 'if
> you are a mdev *and* a iommu then you must want a single PASID' stuff
> in type1.
>
> The group is causing all this mess because the group knows nothing
> about what the device drivers contained in the group actually want.
>
> Further being group centric eliminates the possibility of working in
> cases like !ACS. How do I use PASID functionality of a device behind a
> !ACS switch if the uAPI forces all IOASID's to be linked to a group,
> not a device?
>
> Device centric with an report that "all devices in the group must use
> the same IOASID" covers all the new functionality, keep the old, and
> has a better chance to keep going as a uAPI into the future.
>
> 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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