Re: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory

[David Hildenbrand]

On 05.06.20 11:36, David Hildenbrand wrote:
> On 05.06.20 11:08, David Hildenbrand wrote:
>> On 05.06.20 10:55, Alex Shi wrote:
>>>
>>>
>>> å 2020/1/9 äå9:48, David Hildenbrand åé:
>>>> 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?

Alternative, looks like you can specify "-m 2G,maxmem=16G,slots=1", to
create proper ACPI tables that indicate hotpluggable memory. (I'll have
to look into QEMU to figure out to always indicate hotpluggable memory
that way).


-- 
Thanks,

David / dhildenb