Re: [PATCH 02/10] numa: introduce MachineClass::forbid_asymmetrical_numa

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 20 Aug 2020 12:51:03 -0400
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Aug 20, 2020 at 02:15:04PM +1000, David Gibson wrote:
> > On Wed, Aug 19, 2020 at 10:11:28PM -0400, Eduardo Habkost wrote:  
> > > On Thu, Aug 20, 2020 at 11:17:26AM +1000, David Gibson wrote:  
> > > > On Fri, Aug 14, 2020 at 05:54:16PM -0300, Daniel Henrique Barboza wrote:  
> > > > > The pSeries machine does not support asymmetrical NUMA
> > > > > configurations.  
> > > > 
> > > > This seems a bit oddly specific to have as a global machine class
> > > > property.
> > > > 
> > > > Would it make more sense for machines with specific NUMA constraints
> > > > to just verify those during their initialization?  
> > > 
> > > This would be much simpler.  However, I like the idea of
> > > representing machine-specific configuration validation rules as
> > > data that can eventually be exported to management software.  
> > 
> > Ah, ok, so basically the usual tradeoff between flexibility and
> > advertisability.
> > 
> > So, in that case, I guess the question is whether we envisage "no
> > assymmetry" as a constraint common enough that it's worth creating an
> > advertisable rule or not.  If we only ever have one user, then we
> > haven't really done any better than hard coding the constraint in the
> > manageent software.
> > 
> > Of course to complicate matters, in the longer term we're looking at
> > removing that constraint from pseries - but doing so will be dependent
> > on the guest kernel understanding a new format for the NUMA
> > information in the device tree.  So qemu alone won't have enough
> > information to tell if such a configuration is possible or not.  
> 
> Requiring both QEMU (and possibly management software) to be
> patched again after the guest kernel is fixed sounds undesirable.
If we drop this restriction, then we don't need to touch QEMU when
guest kernel is ready.

Btw, what spapr spec says about the matter?

> Perhaps a warning would be better in this case?
> 
> In either case, it sounds like this won't be a common constraint
> and I now agree with your original suggestion of doing this in
> machine initialization code.
Agreed, if it goes to spapr specific machine code I will not object much.
(it will burden just spapr maintainers, so it's about convincing
David in the end)