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    SubjectRe: [RFC 3/3] mm: iommu: The Virtual Contiguous Memory Manager
    Andi Kleen wrote:

    >>> Also for me it's still quite unclear why we would want this code at all...
    >>> It doesn't seem to do anything you couldn't do with the existing interfaces.
    >> I don't know all that much about what Zach's done here, but from what
    >> he's said so far it looks like this help to manage lots of IOMMUs on a
    >> single system.. On x86 it seems like there's not all that many IOMMUs in
    >> comparison .. Zach mentioned 10 to 100 IOMMUs ..
    > The current code can manage multiple IOMMUs fine.

    That's fair. The current code does manage multiple IOMMUs without issue
    for a static map topology. Its core function 'map' maps a physical chunk
    of some size into a IOMMU's address space and the kernel's address
    space for some domain.

    The VCMM provides a more abstract, global view with finer-grained
    control of each mapping a user wants to create. For instance, the
    symantics of iommu_map preclude its use in setting up just the IOMMU
    side of a mapping. With a one-sided map, two IOMMU devices can be
    pointed to the same physical memory without mapping that same memory
    into the kernel's address space.

    Additionally, the current IOMMU interface does not allow users to
    associate one page table with multiple IOMMUs unless the user explicitly
    wrote a muxed device underneith the IOMMU interface. This also could be
    done, but would have to be done for every such use case. Since the
    particular topology is run-time configurable all of these use-cases and
    more can be expressed without pushing the topology into the low-level
    IOMMU driver.

    The VCMM takes the long view. Its designed for a future in which the
    number of IOMMUs will go up and the ways in which these IOMMUs are
    composed will vary from system to system, and may vary at
    runtime. Already, there are ~20 different IOMMU map implementations in
    the kernel. Had the Linux kernel had the VCMM, many of those
    implementations could have leveraged the mapping and topology management
    of a VCMM, while focusing on a few key hardware specific functions (map
    this physical address, program the page table base register).

    Sent by an employee of the Qualcomm Innovation Center, Inc.
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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