Re: NVDIMM sizes and DIMM hot plug

Peter Krempa <pkrempa(a)redhat.com&gt; 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 >>