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





On 9/10/20 11:59 AM, Daniel P. Berrangé wrote:
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.


I think this approach is cleaner, but I don't mind the previous one (perhaps
tweaking it to update the size in the XML to the auto-aligned value, for
consistency).

I can post patches reverting this change and amending the older approach
(which will not break existing guests). Andrea, what do you think?


Thanks,


DHB




Regards,
Daniel



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