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    SubjectRe: [PATCH] VFIO driver: Non-privileged user level PCI drivers
    On Wed, Jun 02, 2010 at 12:53:12PM +0300, Michael S. Tsirkin wrote:
    > On Wed, Jun 02, 2010 at 11:42:01AM +0200, Joerg Roedel wrote:

    > > IMO a seperate iommu-userspace driver is a nightmare for a userspace
    > > interface. It is just too complicated to use.
    >
    > One advantage would be that we can reuse the uio framework
    > for the devices themselves. So an existing app can just program
    > an iommu for DMA and keep using uio for interrupts and access.

    The driver is called UIO and not U-INTR-MMIO ;-) So I think handling
    IOMMU mappings belongs there.

    > > We can solve the problem
    > > of multiple devices-per-domain with an ioctl which allows binding one
    > > uio-device to the address-space on another.
    >
    > This would imply switching an iommu domain for a device while
    > it could potentially be doing DMA. No idea whether this can be done
    > in a safe manner.

    It can. The worst thing that can happen is an io-page-fault.

    > Forcing iommu assignment to be done as a first step seems much saner.

    If we force it, there is no reason why not doing it implicitly.

    We can do something like this then:

    dev1 = open();
    ioctl(dev1, IOMMU_MAP, ...); /* creates IOMMU domain and assigns dev1 to
    it*/

    dev2 = open();
    ioctl(dev2, IOMMU_MAP, ...);

    /* Now dev1 and dev2 are in seperate domains */

    ioctl(dev2, IOMMU_SHARE, dev1); /* destroys all mapping for dev2 and
    assigns it to the same domain as
    dev1. Domain has a refcount of two
    now */

    close(dev1); /* domain refcount goes down to one */
    close(dev2); /* domain refcount is zero and domain gets destroyed */


    Joerg



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