Re: [Qemu-devel] [PATCH 3/3] pc: Don't make CPU properties mandatory unless necessary

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Aug 16, 2019 at 03:20:11PM +0200, Igor Mammedov wrote:
> On Thu, 15 Aug 2019 15:38:03 -0300
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > We have this issue reported when using libvirt to hotplug CPUs:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1741451
> > 
> > Basically, libvirt is not copying die-id from
> > query-hotpluggable-cpus, but die-id is now mandatory.
> 
> this should have been gated on compat property and affect
> only new machine types.
> Maybe we should do just that instead of fixup so libvirt
> would finally make proper handling of query-hotpluggable-cpus.
> 
>  
> > We could blame libvirt and say it is not following the documented
> > interface, because we have this buried in the QAPI schema
> > documentation:
> 
> I wouldn't say buried, if I understand it right QAPI schema
> should be the authoritative source of interface description.
> 
> If I recall it's not the first time, there was similar issue
> for exactly the same reason (libvirt not passing through
> all properties from query-hotpluggable-cpus).
> 
> And we had to fix it up on QEMU side (numa_cpu_pre_plug),
> but it seems 2 years later libvirt is still broken the same way :(
> 
> Should we really do fixups or finaly fix it on libvirt side?

Is it truly a bug in libvirt?  Making QEMU behave differently
when getting exactly the same input sounds like a bad idea, even
if we documented that at the QAPI documentation.

My suggestion is to instead drop the comment below from the QAPI
documentation.  New properties shouldn't become mandatory.

>  
> > > Note: currently there are 5 properties that could be present
> > > but management should be prepared to pass through other
> > > properties with device_add command to allow for future
> > > interface extension. This also requires the filed names to be kept in
> > > sync with the properties passed to -device/device_add.  
> > 
> > But I don't think this would be reasonable from us.  We can just
> > make QEMU more flexible and let CPU properties to be omitted when
> > there's no ambiguity.  This will allow us to keep compatibility
> > with existing libvirt versions.
> 
> I don't really like making rule from exceptions so I'd suggest doing
> it only for  die_id if we have to do fix it up (with fat comment
> like in numa_cpu_pre_plug).
> The rest are working fine as is.

I will insist we make it consistent for all properties, but I
don't want this discussion to hold the bug fix.  So I'll do this:

I will submit a new patch that makes only die-id optional, and CC
qemu-stable.

After that, i will submit this patch again, and we can discuss
whether we should make all properties optional.

-- 
Eduardo