Two questions about NVDIMM devices

[Daniel Henrique Barboza]

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 at 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 at 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 at gmail.com>
>      Reviewed-by: Andrea Bolognani <abologna at 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