NVDIMM sizes and DIMM hot plug

[Milan Zamazal]

Peter Krempa <pkrempa at redhat.com> writes:

> On Tue, Jun 16, 2020 at 12:54:29 +0200, Milan Zamazal wrote:
>> Hi,
>> 
>
>> I've found out that NVDIMM size and label size matter for regular
>> (non-NV) DIMM hot plug.  If the NVDIMM is not aligned correctly, the
>> guest OS will not accept the hot plugged memory and will complain with
>> messages such as
>> 
>>   Block size [0x8000000] unaligned hotplug range: start 0x225000000, size 0x10000000
>> 
>> The start address above is also reported within <memory> element of the
>> hot plugged memory in the domain XML:
>> 
>>   <address type='dimm' slot='1' base='0x225000000'/>
>> 
>> Apparently, in order to make memory hot plug working in the guest OS,
>> the inserted memory must be aligned to the platform memory alignment
>> (128 MB on x86_64).
>> 
>> I'd like to clarify, how libvirt makes the DIMM address above.  How is
>
> If the address isn't provided in the device XML of the attached device,
> libvirt attaches the device without any address at all and then
> refreshes the address from qemu in 'qemuDomainUpdateMemoryDeviceInfo'.

OK, I can look into the QEMU source code, but I'd still like to have
some official confirmation, especially regarding possible pitfalls or
future changes.  We can't risk data loss.

>> the NVDIMM memory range determined?  According to my experiments, it
>> seems the NVDIMM specified <size> is taken, NVDIMM <label> size is
>> subtracted from it and the resulting value is reduced to the nearest
>> multiple of NVDIMM <alignsize>.  Is this observation correct?  Is it
>> guaranteed to be stable in future versions?  I need to determine the
>> right NVDIMM size to make the subsequent memory modules correctly
>> aligned and then I can't change the NVDIMM size, to not damage data
>> stored in the NVDIMM.
>
>
> Unfortunatelly I didn't implement NVDIMM support so I don't know the
> intricacies. I've cc'd Martin Kletzander who did that part.

Martin, do you know how the QEMU part is supposed to work?  I haven't
received any response on the QEMU list, do you know who could I ask
directly?

>> Additionally, when adjusting maxMemory due to NVDIMM presence, should I
>> increase it by the specified NVDIMM <size> or a different value?
>> 
>> Thank you,
>> Milan
>>