Hi,
I work on NVDIMM support in oVirt/RHV, I think other virtualization
management software built on top of QEMU may have similar concerns.
When a virtual NVDIMM device size is specified, it's not necessarily the
eventual NVDIMM device size visible to the guest OS. As seen in
https://github.com/qemu/qemu/blob/v6.0.0/hw/mem/nvdimm.c#L117, QEMU
makes some adjustments (other adjustments are performed by libvirt but
that's a topic for a different forum):
- NVDIMM label size is subtracted from the NVDIMM size.
- NVDIMM label is pointed to a certain memory region.
- The remaining NVDIMM size is aligned down.
There are some related potential problems:
- If the alignment rules change in a future QEMU version, it may result
in a different device size visible to the guest (even if the requested
size remains the same) and cause trouble there up to data loss.
- If the layout on the backing device changes, e.g. a label placement,
then the stored data may become corrupt or inaccessible.
- I'm not sure about the current QEMU version, but at least in previous
QEMU versions, the resulting size is important for memory hot plug.
The NVDIMM alignment size is smaller than the required regular memory
DIMM placement alignment. If a VM contains an NVDIMM with the
resulting size not matching the DIMM placement requirements and a
memory hot plug is attempted then the hot plug fails because the DIMM
is mapped next to the end of the NVDIMM region, which is not
DIMM-aligned.