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/07de813924caf37e535855541c0c1183... and replacing it with validation in https://gitlab.com/libvirt/libvirt/-/commit/0ccceaa57c50e5ee528f7073fa8723af...
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