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Re: Two questions about NVDIMM devices





On 9/14/20 1:40 PM, Andrea Bolognani wrote:
On Thu, 2020-09-10 at 20:53 +0200, Michal Privoznik wrote:
On 9/10/20 4:56 PM, Daniel P. Berrangé wrote:
On Thu, Sep 10, 2020 at 04:54:08PM +0200, Milan Zamazal wrote:
Daniel P. Berrangé <berrange redhat com> writes:
If we're enforcing this 1 MB rounding though, we really should be
documenting it clearly, so that apps can pick the right backing file
size. I think we dropped the ball on docs.

I still can't see it in the documentation, would it be possible to be
clear about it in the docs, please?  For first, it's not very intuitive
to figure out that (if I've figured out it correctly) on POWER one
*must* specify the NVDIMM size S as

    S == aligned_size + label_size

and that size is used for the QEMU device; while on x86_64 one can
specify any size S and

    align_up(S)

will be used for the QEMU device (and label size doesn't influence the
value).  And additional alignment may be required for having any memory
hot plug working.

The ppc64-specific requirements were documented with

   commit 8f474ceea05aec349be19726e394a62e300efe77
   Author: Daniel Henrique Barboza <danielhb413 gmail com>
   Date:   Mon Jul 20 13:51:46 2020 -0300

     formatdomain.html.in: mention pSeries NVDIMM 'align down' mechanic
The reason why we align down the guest area (total-size - label-size) is
     explained in the body of qemuDomainNVDimmAlignSizePseries(). This
     behavior must also be documented in the user docs.
Signed-off-by: Daniel Henrique Barboza <danielhb413 gmail com>
     Reviewed-by: Andrea Bolognani <abologna redhat com>

but later reverted. See below.

For second, and more importantly, I'm afraid that without documenting
it, future changes may break the current behavior without warning.  For
example, the recent changes regarding POWER alignment in 6.7.0 are for
good IMHO and one can use the same size with both 6.7 and 6.6 versions,
but they could still cause pre-6.7 sizes stop working.

I don't know what changes you are referring to here, but if they were
in libvirt I'd consider that a bug - we shouldn't break a previously
working configuration by increasing required alignment.

I mean disabling the auto alignment in
https://gitlab.com/libvirt/libvirt/-/commit/07de813924caf37e535855541c0c1183d9d382e2
and replacing it with validation in
https://gitlab.com/libvirt/libvirt/-/commit/0ccceaa57c50e5ee528f7073fa8723afd62b88b7

That change can cause a VM fail to start but after (manually) adjusting
the device size, all should work all right.  Changes that would actually
change sizes would be more dangerous.

Sigh, that second commit even calls out the fact that it breaks
existing guests. This needs to be reverted, as that is not acceptable.

Thing is, on PPC it was never working IIRC. I remember discussing this
with Andrea. So from my POV, there wasn't really anything to break.

Yes, this is correct.

However, this is a Libvirt design violation, regardless of whether there are existing
guests to break or not, and this is why I have already agreed with the revert and 'll post
patches soon.



This is my fault for not keeping a close enough eye on the patch
series when it was being posted and reviewed upstream. Sorry :(


Nah. I'm the one that posted the patches ignoring the fact that this was breaking the
intended design. Let's not blame Brno for a Brazilian mess up :P


We'ĺl get this reverted, tidy it up what was there before to make the size consistent
between what QEMU and domain XML sees (without breaking guests) and get it all
wrapped up for the next Libvirt release.



Thanks,


DHB


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