Re: NVDIMM sizes and DIMM hot plug

On Tue, Jun 16, 2020 at 12:54:29PM +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 > 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. Libvirt doesn't ever assign a "base" address value itself. We just start QEMU, and then fill in the XML "base" with the value that QEMU has assigned.

> Additionally, when adjusting maxMemory due to NVDIMM presence, should I > increase it by the specified NVDIMM <size> or a different value? IIRC, maxMemory has to allow for the sum of the basic RAM amount, plus RAM intended to be used for all possible future (NV)DIMMS that will be hotplugged.