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    SubjectRe: Plan for /dev/ioasid RFC v2
    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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