Linux Virtualization - Jun 2020 - [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory

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

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:
>>>
>>>
>>> ? 2020/1/9 ??9:48, David Hildenbrand ??:
>>>> 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

? 2020/6/5 ??5:36, David Hildenbrand ??:
> 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?
Yes, it works fine with this cmdline. problems gone,

? 2020/6/5 ??6:05, David Hildenbrand ??:
>> 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).
> 

That works too. Yes, better resolved in qemu, maybe. :)

Thanks!