Re: Adjustments of NVDIMM devices and future data safety

["Liu, Jingqi" <jingqi.liu@intel.com>]

Hi Milan,

On 4/30/2021 8:18 PM, Milan Zamazal wrote:
> 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.

Can you explain the details and give an example of how to reproduce this 
issue ?

Thanks,

Jingqi