Re: NVDIMM sizes and DIMM hot plug

[Martin Kletzander <mkletzan@xxxxxxxxxx>]

On Thu, Jun 18, 2020 at 11:22:46AM +0200, Milan Zamazal wrote:
> Peter Krempa <pkrempa@xxxxxxxxxx> 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?

Sorry, no idea, it was actually Michal (Cc'd) who implemented it if I'm not
mistaken.

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

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