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    SubjectRe: swiotlb detection should be memory hotplug aware ?
    , Alok Kataria wrote:

    Hi Alok,

    > Hi,
    > Looking at the current code swiotlb is initialized for 64bit kernels
    > only when the max_pfn value is greater than 4G (MAX_DMA32_PFN value).
    > So in cases when the initial memory is less than 4GB the kernel boots
    > without enabling swiotlb, when we hotadd memory to such a kernel and go
    > beyond the 4G limit, swiotlb is still disabled. As a result when any
    > 32bit devices start using this newly added memory beyond 4G, the kernel
    > starts spitting error messages like below or in some cases it causes
    > kernel panics.

    Yes seems like a real problem.

    > 1. Enable swiotlb for all 64bit kernels which have memory hot-add
    > support.

    I don't think that's a good idea. It would enable it everywhere on
    distributions which compile with hotadd. Need (2)

    > 2. Instead of checking the max_pfn value in pci_swiotlb_detect, check
    > for max_hotpluggable_pfn (or some such) value. Though I don't see such a
    > value readily available. I could parse the SRAT and get hotplug memory
    > information but that will make swiotlb detection logic a little too
    > complex. A quick look around srat_xx.c files and the acpi_memhotplug
    > module didn't find any useful API that could be used directly either.
    > So was wondering if any of you are aware of an easy way to get such
    > information ?

    I have a patchkit to revamp the SRAT parsing to store the hotadd information
    more efficiently (the current way is pretty dumb) I need to repost that.

    With that it would be relatively easy to do I think.


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