[PATCH 2/5] qemu_validate.c: add pSeries NVDIMM size alignment validation

[Daniel P. Berrangé]

On Thu, Jul 30, 2020 at 04:48:01PM -0300, Daniel Henrique Barboza wrote:
> The existing auto-align behavior for pSeries has the idea to
> alleviate user configuration of the NVDIMM size, given that the
> alignment calculation is not trivial to do (256MiB alignment
> of mem->size - mem->label_size value, a.k.a guest area). We
> align mem->size down to avoid end of file problems.
> 
> The end result is not ideal though. We do not touch the domain
> XML, meaning that the XML can report a NVDIMM size 255MiB smaller
> than the actual size the guest is seeing. It also adds one more
> thing to consider in case the guest is reporting less memory
> than declared, since the auto-align is transparent to the
> user.
> 
> Following Andrea's suggestion in [1], let's instead do an
> size alignment validation. If the NVDIMM is unaligned, error out
> and suggest a rounded up value. This can be bothersome to users,
> but will bring consistency of NVDIMM size between the domain XML
> and the guest.
> 
> This approach will force existing non-running pSeries guests to
> readjust the NVDIMM value in their XMLs, if necessary. No changes
> were made for x86 NVDIMM support.

This change violates on of the key requirements of libvirt design
which is to never break existing guests that are running successfully.
We must not assume users are ok with manually changing their guests
during an upgrade scenario, so IMHO we need to revert this change.


Regards,
Daniel
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