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    SubjectRe: [PATCH] [ppc64] watch IOMMU virtual merging

    > This is a rather painful state of affairs. I'm not convinced external
    > fragmentation in the IOMMU address space is insurmountable as the
    > physically contiguous segments can (in principle; the mechanics of
    > ramming this through the IO subsystem are another matter entirely) be
    > IO-mapped in a bus-discontiguous fashion.

    Yep they can, but if the SG list has been sized to fit after physically
    merging then we have a problem when we later on want to split it. If the
    architecture had more control over the merging process we could do a
    better job here.

    > I'm not familiar with the TCE space regions; could you describe or
    > point to documentation for the semantics there?

    Think of it as a mapping from PCI to host memory, a window of at least
    128MB and up to 3GB. You get to carve it up on a page granularity as you
    like, we currently create 2 regions, a small allocation region and a
    large one (above 15 pages). The aim is to (hopefully) avoid
    fragmentation in the large allocation region.

    > My first thought is to artificially limit the amount of physical
    > merging (hopefully to some nonzero amount instead of disabling it
    > entirely) allowed to take place in order to allow for better virtual
    > merging.

    Removing the large allocation region should also fix our problems. I
    should test that to verify its our problem.

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