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    SubjectRe: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory
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    On 05.06.20 11:08, David Hildenbrand wrote:
    > On 05.06.20 10:55, Alex Shi wrote:
    >>
    >>
    >> 在 2020/1/9 下午9:48, David Hildenbrand 写道:
    >>> Ping,
    >>>
    >>> I'd love to get some feedback on
    >>>
    >>> a) The remaining MM bits from MM folks (especially, patch #6 and #8).
    >>> b) The general virtio infrastructure (esp. uapi in patch #2) from virtio
    >>> folks.
    >>>
    >>> I'm planning to send a proper v1 (!RFC) once I have all necessary MM
    >>> acks. In the meanwhile, I will do more testing and minor reworks (e.g.,
    >>> fix !CONFIG_NUMA compilation).
    >>
    >>
    >> Hi David,
    >>
    >> Thanks for your work!
    >>
    >> I am trying your https://github.com/davidhildenbrand/linux.git virtio-mem-v5
    >> which works fine for me, but just a 'DMA error' happens when a vm start with
    >> less than 2GB memory, Do I missed sth?
    >
    > Please use the virtio-mem-v4 branch for now, v5 is still under
    > construction (and might be scrapped completely if v4 goes upstream as is).
    >
    > Looks like a DMA issue. Your're hotplugging 1GB, which should not really
    > eat too much memory. There was a similar issue reported by Hui in [1],
    > which boiled down to wrong usage of the swiotlb parameter.
    >
    > In such cases you should always try to reproduce with hotplug of a
    > sam-sized DIMM. E.g., hotplugging a 1GB DIMM should result in the same
    > issue.
    >
    > What does your .config specify for CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE?
    >
    > I'll try to reproduce with v4 briefly.

    I guess I know what's happening here. In case we only have DMA memory
    when booting, we don't reserve swiotlb buffers. Once we hotplug memory
    and online ZONE_NORMAL, we don't have any swiotlb DMA bounce buffers to
    map such PFNs (total 0 (slots), used 0 (slots)).

    Can you try with "swiotlb=force" on the kernel cmdline?

    --
    Thanks,

    David / dhildenb

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