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    SubjectRe: [PATCH] VFIO driver: Non-privileged user level PCI drivers
    * Joerg Roedel (joro@8bytes.org) wrote:
    > On Wed, Jun 02, 2010 at 02:21:00PM +0300, Michael S. Tsirkin wrote:
    > > On Wed, Jun 02, 2010 at 01:12:25PM +0200, Joerg Roedel wrote:
    >
    > > > Even if it is bound to a domain the userspace driver could program the
    > > > device to do dma to unmapped regions causing io-page-faults. The kernel
    > > > can't do anything about it.
    > >
    > > It can always corrupt its own memory directly as well :)
    > > But that is not a reason not to detect errors if we can,
    > > and not to make APIs hard to misuse.
    >
    > Changing the domain of a device while dma can happen is the same type of
    > bug as unmapping potential dma target addresses. We can't catch this
    > kind of misuse.
    >
    > > > > With 10 devices you have 10 extra ioctls.
    > > >
    > > > And this works implicitly with your proposal?
    > >
    > > Yes. so you do:
    > > iommu = open
    > > ioctl(dev1, BIND, iommu)
    > > ioctl(dev2, BIND, iommu)
    > > ioctl(dev3, BIND, iommu)
    > > ioctl(dev4, BIND, iommu)
    > >
    > > No need to add a SHARE ioctl.
    >
    > In my proposal this looks like:
    >
    >
    > dev1 = open();
    > ioctl(dev2, SHARE, dev1);
    > ioctl(dev3, SHARE, dev1);
    > ioctl(dev4, SHARE, dev1);
    >
    > So we actually save an ioctl.

    This is not any hot path, so saving an ioctl shouldn't be a consideration.
    Only important consideration is a good API. I may have lost context here,
    but the SHARE API is limited to the vfio fd. The BIND API expects a new
    iommu object. Are there other uses for this object? Tom's current vfio
    driver exposes a dma mapping interface, would the iommu object expose
    one as well? Current interface is device specific DMA interface for
    host device drivers typically mapping in-flight dma buffers, and IOMMU
    specific interface for assigned devices typically mapping entire virtual
    address space.

    thanks,
    -chris


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