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

[David Hildenbrand]

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?

-- 
Thanks,

David / dhildenb