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    SubjectRe: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices
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    On 10/27/2016 12:50 PM, Alexey Kardashevskiy wrote:
    > On 18/10/16 08:22, Kirti Wankhede wrote:
    >> VFIO IOMMU drivers are designed for the devices which are IOMMU capable.
    >> Mediated device only uses IOMMU APIs, the underlying hardware can be
    >> managed by an IOMMU domain.
    >>
    >> Aim of this change is:
    >> - To use most of the code of TYPE1 IOMMU driver for mediated devices
    >> - To support direct assigned device and mediated device in single module
    >>
    >> Added two new callback functions to struct vfio_iommu_driver_ops. Backend
    >> IOMMU module that supports pining and unpinning pages for mdev devices
    >> should provide these functions.
    >> Added APIs for pining and unpining pages to VFIO module. These calls back
    >> into backend iommu module to actually pin and unpin pages.
    >>
    >> This change adds pin and unpin support for mediated device to TYPE1 IOMMU
    >> backend module. More details:
    >> - When iommu_group of mediated devices is attached, task structure is
    >> cached which is used later to pin pages and page accounting.
    >
    >
    > For SPAPR TCE IOMMU driver, I ended up caching mm_struct with
    > atomic_inc(&container->mm->mm_count) (patches are on the way) instead of
    > using @current or task as the process might be gone while VFIO container is
    > still alive and @mm might be needed to do proper cleanup; this might not be
    > an issue with this patchset now but still you seem to only use @mm from
    > task_struct.
    >

    Consider the example of QEMU process which creates VFIO container, QEMU
    in its teardown path would release the container. How could container be
    alive when process is gone?

    Kirti

    >
    >
    >> - It keeps track of pinned pages for mediated domain. This data is used to
    >> verify unpinning request and to unpin remaining pages while detaching, if
    >> there are any.
    >> - Used existing mechanism for page accounting. If iommu capable domain
    >> exist in the container then all pages are already pinned and accounted.
    >> Accouting for mdev device is only done if there is no iommu capable
    >> domain in the container.
    >> - Page accouting is updated on hot plug and unplug mdev device and pass
    >> through device.
    >>
    >> Tested by assigning below combinations of devices to a single VM:
    >> - GPU pass through only
    >> - vGPU device only
    >> - One GPU pass through and one vGPU device
    >> - Linux VM hot plug and unplug vGPU device while GPU pass through device
    >> exist
    >> - Linux VM hot plug and unplug GPU pass through device while vGPU device
    >> exist
    >>
    >> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
    >> Signed-off-by: Neo Jia <cjia@nvidia.com>
    >> Change-Id: I295d6f0f2e0579b8d9882bfd8fd5a4194b97bd9a
    >
    >

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